Preheat your oven to 350°F and coat a 9x13 baking dish with nonstick spray.
Brown the sausage in a large skillet over medium-high heat until crumbled and cooked through.
In the same skillet, sauté onions and peppers until softened (about 6 minutes), then add spinach and cook until just wilted.
Transfer the sausage and veggie mixture to your prepared baking dish.
In a large bowl, whisk together eggs, cream, salt, pepper, and dried mustard. Stir in the shredded cheese.
Pour the egg mixture over the sausage and veggies, stirring to combine evenly.
Bake for 35-40 minutes until set and lightly golden on top.