Melt butter in pan. Add onions, garlic and mushrooms. Sautee until onions are soft.
Meanwhile cook egg noodles according to package directions, drain.
Add flour to butter and onion mixture and mix thoroughly over medium high heat until well blended. Gradually pour in milk. Add in salt & pepper and cream cheese. Heat until cream cheese is melted and mixture is thick and bubbly. Add more flour paste if needed.
Mix together sauce, noodles, tuna, and peas and turn out into a 9x13 casserole.
Top with cheddar cheese and sprinkle with French fried onions.