Preheat Oven and Prep Baking Dish: Preheat oven to 400 degrees F and coat a 9x13-inch baking dish with nonstick cooking spray.

Cook Sausage: To a large pan over medium-high heat, add sausage and break apart with a wooden spoon. Cook sausage until browned, about 5 minutes. Remove pan from heat and, using a slotted spoon, transfer browned sausage to a prepared baking dish and set aside. (Alternatively, carefully drain grease into a jar or container and then pour browned sausage into baking dish.)

Prepare Egg Mixture: To a large bowl, add eggs, shredded cheddar, shredded mozzarella, heavy cream, salt, and pepper. Using an electric mixer, mix until well-combined. Pour egg mixture atop browned sausage in baking dish.

Bake Casserole: Transfer baking dish to middle oven rack and bake until edges of casserole are golden brown and bubbling, about 30-35 minutes.

Slice and Serve: Remove baking dish from oven and allow casserole to cool slightly before slicing into pieces. Garnish with sprinkle of chopped fresh chives.