Heat the oven to 375°F. Lightly coat a 9x13-inch baking dish with cooking spray or oil.

Heat 1 tablespoon olive oil in a large skillet over medium heat until shimmering. Add 1 pound uncooked pork breakfast sausage and cook, breaking it up into small pieces with a wooden spoon, until no longer pink, 6 to 8 minutes.

Stir in 1 diced medium yellow onion, 1 diced medium red bell pepper, and ½ teaspoon of the kosher salt. Cook, stirring occasionally, until the onion is softened, 3 to 5 minutes. Stir in 3 ounces baby spinach. Cook until wilted, about 1 minute. Transfer to the baking dish.

Whisk 12 large eggs, 2 cups half-and-half, 1 tablespoon Dijon mustard, the remaining 1 teaspoon kosher salt, and ½ teaspoon black pepper in a large bowl until combined.

Add 20 ounces frozen shredded potatoes (if it’s frozen into big clumps, smash the unopened bag against a hard surface to break them up first) and half of the shredded sharp cheddar cheese (about ¾ cup) to the baking dish. Stir until combined and spread into an even layer.

Pour the egg mixture over the potato mixture. Tilt the baking dish so the egg mixture is in an even layer if needed. Sprinkle with the remaining ¾ cup cheddar.

Bake until a knife inserted in the middle comes out clean and the top is light golden-brown, 55 to 60 minutes. Let cool for 5 minutes before serving.