Cheesy Sausage Breakfast Casserole

Morning magic happens when sausage, cheese, and eggs unite in one spectacular dish! This breakfast casserole isn’t just a meal—it’s the reason to jump out of bed on weekend mornings. The combination of savory sausage, earthy mushrooms, and melty cheese creates that irresistible comfort food experience that’ll have everyone asking for seconds.

What makes this casserole truly special is its perfect balance of textures and flavors. The crispy-edged sausage brings richness, while sun-dried tomatoes add pops of intense sweetness. Spinach lightens everything up, and that custard-like egg mixture binds it all together with creamy decadence. Plus, it’s adaptable—make it vegetarian by skipping the sausage or swap in your favorite cheese to create your perfect breakfast masterpiece.

Ingredients

• 2 tablespoons extra-virgin olive oil, plus more as needed
• 12 ounces (340g) bulk sweet or hot Italian sausage (optional)
• 5 ounces (140g) fresh mushrooms (shiitake, cremini, or oyster), thinly sliced (2 cups)
• 1 small red onion (6 ounces; 170g), thinly sliced (about 1 cup)
• 1½ teaspoons Diamond Crystal kosher salt, divided (for table salt, use half as much)
• ½ cup (3 ounces; 85g) sun-dried tomatoes in oil, drained and chopped
• 3 medium garlic cloves, minced
• 5 ounces (140g) fresh baby spinach (5 cups)
• 6 ounces (170g) Gouda or cheddar cheese, shredded (1½ cups)
• 10 large eggs
• ⅓ cup (80ml) sour cream or plain, whole-milk Greek yogurt
• 1½ cups (360ml) half-and-half or whole milk
• 1 tablespoon (15ml) Dijon mustard

Steps

1. Position rack in middle position and preheat oven to 350°F (175°C). Lightly grease a 9 × 13-inch baking dish with oil, making sure to coat sides to prevent sticking; set aside. If using sausage, heat 1 tablespoon oil in a 12-inch skillet over medium-high until shimmering (oil should flow easily but not smoke). Add sausage and break into large clumps with a wooden spoon. Let sausage cook undisturbed for 4 minutes until deeply browned on bottom—this creates flavorful caramelization. Break meat into smaller pieces and continue cooking, stirring occasionally, for about 3 minutes until just cooked through with no pink remaining. Transfer sausage to paper towel–lined plate using a slotted spoon. Reserve 2 tablespoons rendered fat in skillet for cooking mushrooms.
2. Heat reserved fat (or 2 tablespoons oil if omitting sausage) in the same skillet over medium-high heat until shimmering. Add mushrooms, stir to coat evenly in oil, then spread in a single layer. Cook without disturbing for 4 minutes until bottoms develop a golden-brown sear—moving them too soon prevents proper caramelization. Reduce heat to medium, then add onion, ½ teaspoon salt, and 1 tablespoon oil. Cook, stirring occasionally, for about 8 minutes until onions turn translucent and mushrooms are browned all over.
3. Add sun-dried tomatoes and garlic to the skillet; cook, stirring frequently, for about 1 minute until fragrant—watch carefully as garlic burns easily. Add spinach in 2 batches, stirring constantly and waiting for the first batch to wilt completely (about 1-2 minutes) before adding the second batch. The spinach will reduce dramatically in volume. Transfer the complete vegetable mixture and sausage (if using) to the prepared baking dish. Add shredded cheese and stir until evenly distributed, then spread mixture in an even layer.
4. In a large bowl, whisk eggs vigorously until completely uniform with no visible streaks of white or yolk. Whisk in sour cream in 3 separate additions, ensuring each addition is fully incorporated before adding the next—this prevents lumps that would create an uneven texture. Whisk in half-and-half, Dijon mustard, and remaining 1 teaspoon salt until thoroughly combined. Pour egg mixture over the mushroom mixture in baking dish, then gently stir to ensure egg mixture reaches all corners and ingredients are evenly distributed.
5. Bake, uncovered, for 35 minutes until casserole is puffed, set in the center, and lightly golden on top. To test doneness, insert a knife in the center—it should come out clean with no runny egg mixture. If the top browns too quickly but center isn’t set, tent loosely with foil. Let cool for 10 minutes before slicing—this resting period allows the eggs to set completely for cleaner slices. Serve warm.

Smart Swaps

• Pepper jack cheese instead of Gouda for a spicy kick (same 1:1 ratio)
• Turkey sausage instead of Italian sausage to reduce fat while maintaining flavor
• Almond milk instead of half-and-half for a lighter, dairy-free option (use same 1½ cups)
• Kale instead of spinach for a sturdier green that holds up well during baking

Make It Diabetes-Friendly

• Replace half-and-half with unsweetened almond milk to reduce carbs by approximately 8g per serving
• Use turkey breakfast sausage instead of Italian sausage to reduce fat content by about 30%
• Increase vegetable content by adding 1 cup diced bell peppers while reducing cheese to 4 ounces (1 cup) to lower overall calorie density
• Serve with a side of fresh berries instead of toast or potatoes to keep the glycemic impact low
• Consider portion control: a 3×3-inch square provides adequate protein without excessive carbs

Pro Tips

• Assemble the entire casserole the night before, cover tightly with plastic wrap, and refrigerate. Add an extra 5 minutes to baking time when cooking from cold.
• For extra flavor, add ½ teaspoon dried herbs like thyme or oregano to the vegetable mixture
• Shred cheese yourself rather than using pre-shredded for better melting (pre-shredded contains anti-caking agents)
• Let ingredients come to room temperature before baking for more even cooking
• For a crispy top, sprinkle an additional ¼ cup cheese over casserole during final 5 minutes of baking