Step 1

Preheat oven to 400°F. Prepare a 9x13-inch baking dish with nonstick spray. Set aside.

Step 2

Over a medium heat, melt butter in a medium skillet. Add onions and cook until soft and browning, about 6-8 minutes.

Step 3

Next sprinkle flour over onions and cook for about a minute, stirring constantly. Slowly stir in milk. Continue to cook, stirring frequently, until sauce starts to bubble and slightly thicks. Stir in salt, pepper, and garlic powder.

Step 4

Remove from heat and stir in 1 cup of cheddar cheeses until melted.

Step 5

In a large mixing bowl, add in your cooked pasta, tuna and capers. Pour cheese sauce over pasta and mix until well combined. Transfer mixture to baking dish. Layer provolone slices on top of casserole. Next top with tomato slices, and remaining cheese. Sprinkle dill on top of cheese.

Step 6

Cover with foil and bake for 20 minutes, removing the foil for the last 5 minutes.