Prepare the noodles according to the package directions. Drain, then transfer to a large bowl, drizzle with 1 teaspoon of oil, and toss to avoid clumping.
Preheat the oven to 375°F. Butter a medium casserole.
Melt the butter in a large saucepan over medium heat, add the onion, and cook for 5 minutes, or until soft. Add the flour, seafood seasoning, and paprika and cook for 1 minute, stirring constantly. Add the milk, salt, and mustard and whisk to blend. Cook for 3 to 5 minutes, or until the sauce thickens, whisking often. Add the sour cream and whisk until blended. Pour the sauce over the noodles.
Add the tuna and its oil, tomato, and cheddar cheese and mix to coat. Pour the mixture into the casserole. Sprinkle with the cracker crumbs, if using, and drizzle with the remaining olive oil. Bake for 30 minutes, or until bubbly.