Heat a large fry pan over medium heat. Saute mushrooms, garlic, and onion in fry pan until onions are softened and set aside.
While the onion and mushroom mixture is sauteing, cook the 12 oz. large bag of noodles in salted water until tender. Drain and set aside.
Melt butter in large fry pan. Add flour and whisk together to make a roux. Add chicken broth and heavy cream, slowly, stirring with the whisk constantly. Add 2 teaspoon brown mustard.
When sauce is thick and smooth, add cooked noodles, tuna, peas, and the onion and mushrooms mixture. Season with salt and pepper to taste. Pour into a large 9 x 13 greased baking dish.
Top with cheddar cheese and then sprinkle crunched up French onions.
Bake at 375 degrees for 20 minutes or until casserole is slightly browned over the top.