Preheat oven to 350°. Prepare a 9 x 13-inch baking dish. In a skillet, melt 1 tablespoon of butter over medium heat; add onions and ½ teaspoon salt, and sauté until onions are soft and slightly brown, about 10 minutes. Remove from skillet and set aside.

Using the same skillet, brown ground chuck and sirloin over medium heat until thoroughly cooked. Sprinkle beef with 1½ teaspoons salt; add 1 cup of cooked onions and stir. Set aside.

In a saucepan, melt 2 tablespoons of butter over medium heat. Add flour and whisk until incorporated. Slowly add milk and whisk until mixture thickens. Reduce heat to low and stir in tomato paste, water, mustard, garlic powder, brown sugar, Worcestershire sauce, and black pepper. Cook on low for 15 minutes, stirring occasionally.

In a large mixing bowl, combine cooked macaroni, ground beef and onion mixture, 1 cup of shredded cheese, and sauce. Pour mixture into baking dish and bake for 30 minutes. Top with remaining cheese and onions and continue baking for 15 minutes or until cheese is melted and casserole is bubbling. Remove from oven and serve.