1Lay bread on a work surface and spread butter on one side of each slice.
2Layer bacon, 1 slice of cheese, onion rings, avocado slices, and another slice of cheese on top of the unbuttered-side of the bread; top with second slice of bread, buttered-side facing up.
3Cook sandwiches in a skillet over medium heat, flipping once, until both sides are golden brown and cheese has melted, about 3 to 5 minutes per side.