1. Preheat your oven to 375°F.
2. Begin by peeling the onions and slicing them into thin rounds.
3. Place the sliced onions in a greased 9×13 inch baking dish.
4. Add the sliced butter to the baking dish, spreading it out evenly across the bottom of the dish.
5. Place the baking dish in the preheated oven for 5-7 minutes, or until the butter is melted and bubbly.
6. In a separate bowl, whisk together the flour and milk until well combined.
7. Add the garlic powder, onion powder, paprika, salt, and pepper to the bowl and whisk again to combine.
8. Pour the milk and flour mixture over the sliced onions in the baking dish, making sure the onions are evenly coated.
9. Sprinkle the shredded cheddar cheese over the top of the onion mixture.
10. Bake the casserole in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
11. Allow the casserole to cool for a few minutes before serving.
