Combine the onion, oil, chopped up tuna, and mayonnaise.
Spread margarine on the bread and then spread the tuna-mayo mixture over that. Put a slice of cheese on top and toast in an oven until melted.
My husband likes it when I make this inside of an English muffin.
Story Behind this Recipe
I usually eat onion and tuna-mayo in a sandwich, but my daughter likes it with cheese and for it to be toasted. The texture is best when the onion is finely chopped, but if that's a pain, you don't have to~ It also tastes great on English muffin.