Preheat Oven and Prep Baking Dish: Preheat oven to 400°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 the 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 and the center is set and no longer jiggles, about 30 to 35 minutes.

Slice and Serve: Remove baking dish from oven and let casserole rest for about 5 minutes before slicing into 9 pieces. Garnish with a sprinkle of chopped fresh chives.