Preheat the oven: Set your oven to 350°F (175°C). Grease a 9×13-inch baking dish to prevent sticking and ensure easy serving.
Cook the sausage and onion: In a large skillet over medium heat, cook the sausage and chopped onion until the sausage is browned and the onion is tender. Stir occasionally to combine the flavors.
Drain excess fat: Remove any excess grease from the skillet to keep the casserole from becoming too oily.
Combine ingredients: In a large mixing bowl, add the cooked sausage and onions, thawed hash browns, shredded cheddar, cottage cheese, and sour cream. Mix thoroughly to distribute all the ingredients evenly.
Season: Add salt and pepper to taste. Be cautious with the salt, as the sausage and cheeses already contain some.
Spread in dish: Evenly spread the mixture into the prepared baking dish. An even layer ensures consistent baking and texture.
Bake: Place the dish in the preheated oven and bake for 45-50 minutes, or until the top is golden brown and the edges are bubbly. The golden crust adds a crunchy texture and indicates the casserole is cooked through.
Garnish and serve: Let the casserole rest for a few minutes before serving. Garnish with chopped green onions for a pop of freshness and color.