Preheat oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray.
In a medium skillet over medium heat, add the olive oil. Once hot, add the diced onion and cook until translucent, about 10-12 minutes. Set aside.
In a medium pot over medium heat melt the butter. Add flour, salt, and pepper and whisk for 1-2 minutes, or until golden brown in color.
While whisking constantly, slowly pour in the milk. Bring the sauce to a boil.
When sauce starts to boil, turn the heat to low. Continue whisking constantly until the sauce is thickened.
Remove from heat and add 1 cup parmesan cheese and 1 cup cheddar cheese. Stir until the cheeses are melted.
In a large bowl, add tuna, sweet corn, creamed corn, sautéed onions, and the cooked noodles. Stir to combine.
Pour the cheese sauce onto the tuna mixture. Stir to combine.
Transfer the casserole to the prepared baking dish. Top with remaining parmesan cheese.
Bake for 20-25 minutes, or until golden brown.