Preheat the oven to 400 degrees F. Lightly grease a 9 x13-inch casserole dish or 12-inch Dutch oven.
Cook the noodles according to the directions on the package.
Meanwhile, melt the butter in a large pot over medium heat. Stir in the onions and cook for about 5 minutes, or until softened.
Stir in the garlic and flour and cook for 1 minute, stirring constantly. Slowly stir in the chicken broth and cream. Bring to a simmer for about 3 minutes, or until the mixture thickens slightly, stirring frequently.
Stir in the tuna, cheddar and Monterey Jack cheeses. Fold in the pasta until coated well.
Pour the mixture into the casserole dish or Dutch oven.
In a small bowl, mix together the cracker crumbs, Parmesan and oil. Evenly sprinkle over the pasta. Sprinkle with additional Parmesan cheese.
Bake for about 20 minutes, or until the bread crumbs are lightly browned and the casserole is bubbling through. Serve hot.