Preheat oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.
Place sausage in a large skillet and pour in about 1/4 inch of water; cook sausage over medium heat until water evaporates and sausages are evenly browned, 10 to 15 minutes. Crumble cooked sausages into a bowl.
Mix sweet potatoes and butter together in a bowl; evenly spread into the bottom of the prepared 9x13-inch dish.
Stir Cheddar-mozzarella cheese blend, onion, spinach, cottage cheese, eggs, and crumbled sausage together in a large bowl; spoon over sweet potato layer.
Bake casserole in the preheated oven until a toothpick inserted in the center comes out clean and eggs are set, about 1 hour. Cool 5 minutes before serving.