Heat the oven to 350°F (175°C). Coat a 9 × 13-inch baking dish with butter or cooking spray, then set it aside.
Place the diced bacon in a large skillet and cook over medium heat until crisp. Add the diced onion and continue cooking until the onion turns translucent. Remove the skillet from the heat, then transfer the bacon and onion to paper towels to drain away the excess grease.
Crack the eggs into a large mixing bowl. Add the salt, black pepper, and hot sauce, if using, and whisk until thoroughly blended.
Add the thawed hash browns, shredded sharp cheddar, shredded Swiss cheese, small-curd cottage cheese, cooked bacon, and softened onion to the egg mixture. Stir until all components are evenly distributed.
Transfer the mixture to the prepared 9 × 13-inch baking dish and spread it into an even layer. Bake in the preheated oven for 35–45 minutes, or until the center is completely set and no longer jiggles.
Remove the casserole from the oven and allow it to stand for 5–10 minutes before cutting. Garnish with chopped fresh chives, if desired, and serve.