Heat a skillet over medium-high heat and add the sausage, breaking it into small crumbles as it cooks. Once browned and cooked through (about 5-7 minutes), transfer to a paper towel-lined plate to drain excess fat. While the sausage cooks, dice the onion and bell pepper into roughly 1/2-inch pieces so everything is ready to layer. I prefer to cook the sausage first so any fat can drain away, keeping the quiche from becoming greasy.

In a large mixing bowl, crack the room temperature eggs and whisk them together until well combined and slightly frothy. Add the whipping cream and continue whisking until the mixture is smooth and uniform. Using room temperature eggs helps them blend more smoothly and cook more evenly throughout the quiche, resulting in a creamier texture.

Preheat the oven to 325°F. Spray a 9-inch pie dish or similar baking dish with cooking spray to prevent sticking. Layer the ingredients in this order: first sprinkle the shredded mozzarella evenly across the bottom, then add the browned sausage from Step 1, followed by the diced onions and bell peppers. This layering ensures the cheese anchors everything in place and creates a nice base. Pour the egg and cream mixture from Step 2 over all the ingredients, making sure it flows into all the gaps and covers everything evenly.

Bake at 325°F for 1 hour until the custard is set but still slightly jiggly in the very center—this carryover cooking will finish it perfectly. The top should be lightly golden and a knife inserted near the center should come out mostly clean with just a tiny bit of custard clinging to it. Let the quiche rest for 5-10 minutes after removing from the oven before slicing, which helps it hold together better and makes serving easier.