Note: If desired, add additional cheese to the top of the casserole before baking.
Cook noodles according to instructions on the package or al dente.
In a medium skillet add oil and sauté onions and mushrooms over medium-low heat until mushrooms begin to release moisture, about 10 minutes. In a large mixing bowl, combine sautéed onions and mushrooms and all remaining ingredients, reserving 1/2 cup cheddar cheese for the top.
Add the tuna mixture to a 3 quart casserole dish or 9" baking pan, top with the 1/2 cup of remaining cheddar cheese. Bake 15 minutes covered and 10 minutes uncovered, or until cheese is hot and bubbly.