Preheat oven to 425 degrees.
Cook noodles in boiling, salted water until done. Drain and rinse with cold water. Set aside.
In a small bowl, combine flour, chicken bouillon, beef bouillon and garlic. Set aside.
In a large fry pan or Dutch oven, heat a small amount of oil over medium heat and sauté onions until tender. About halfway through, add celery and mushrooms and continue to cook until tender.
Add in the dry seasoning mix, and cook, stirring constantly, until all is mixed well, approximately one minute. Be careful not to burn.
Slowly add in the milk, stirring well and making sure nothing is sticking to the bottom of the pan. Turn the heat to low and cook until the sauce thickens.
Add the mayonnaise, one cup shredded cheese, mixed vegetables and tuna. Mix well.
Once the cheese has melted, remove the pan from the heat. Fold in cooked noodles.
Spray a 10x15-inch lasagne pan with non-stick cooking spray. A 9x12-inch pan can be used in a pinch, but it is a tight fit. Place the noodle mixture into the prepared pan.
Top the casserole with the remaining shredded cheese, 3 cups are preferred. Alternatively, you can substitute buttered gluten-free bread crumbs.
Bake for approximately 40 minutes until the top of the casserole is a nice golden brown. Remove from oven and let rest for a few minutes to make serving easier.
Substitutions: Leftover cooked chicken or turkey can be used in place of tuna. This recipe also lends itself well to using leftover cooked fresh vegetables that may be hiding in the refrigerator.