Thinly slice the onion and prepare the bacon by pan-frying both sides and slicing it.

Heat a pan, add oil, and sauté the onion until softened.

When the onion is almost cooked, add the bacon slices and sauté briefly before turning off the heat.

Spread butter on one side of each slice of bread.

On the unbuttered side of one slice, layer the sautéed onion and bacon mixture.

Add mozzarella cheese on top of the onion and bacon mixture, then cover with the other slice of bread, buttered side facing up.