Cheesy Sausage Breakfast Casserole

Instructions

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1. 1 Adjust oven rack to middle position and preheat oven to 350°F (175℃). Lightly grease a 9- x 13-inch baking dish with oil; set aside. If using sausage, in a 12-inch skillet, heat 1 tablespoon oil over medium-high until shimmering. Add sausage and using a wooden spoon, break it into large clumps. Continue to cook without moving until browned on bottom, about 4 minutes. Use wooden spoon to break up meat into smaller pieces and continue to cook, stirring occasionally, until sausage is just cooked through, about 3 minutes. Using a slotted spoon, transfer sausage to a paper towel–lined plate. Reserve 2 tablespoons rendered fat in skillet to cook mushrooms.
2. 2 Heat rendered fat in skillet (or if omitting sausage, heat 2 tablespoons oil in 12-inch skillet) over medium-high heat until shimmering. Add mushrooms, stir to coat in oil, and spread in an even layer. Cook without moving until bottoms of mushrooms are browned, about 4 minutes. Reduce heat to medium. Add onion, 1/2 teaspoon salt, and 1 tablespoon oil. Cook, stirring occasionally, until onions soften and mushrooms are browned all over, about 8 minutes.
3. 3 Add sundried tomatoes and garlic; cook, stirring often, until fragrant, about 1 minute. Add spinach in 2 additions, stirring often and letting spinach completely wilt before adding more. Transfer vegetable mixture, and sausage (if using), to prepared baking dish. Stir in cheese until combined, and spread mixture in an even layer.
4. 4 In a large bowl, whisk eggs until no streaks remain. Whisk in sour cream in 3 additions until no lumps remain. Whisk in half-and-half, mustard, and remaining 1 teaspoon salt until combined. Pour egg mixture over mushroom mixture in baking dish; gently stir until egg mixture is evenly distributed.
5. 5 Bake, uncovered, until cooked through, puffed and set, about 35 minutes. Let cool 10 minutes. Slice and serve.