Melt butter in Saucepan. Slice mushrooms and cut up peppers, add to butter and cook over medium head ~5 minutes. Remove from heat. Blend in flour, salt and pepper. Cook over low heat, stirring constantly until mixture is bubbly. Remove from heat. Stirr in instant bouillon, milk and water. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in tuna (drained and flaked). Serve over toast.