1. Preheat oven, doesn’t matter what temperature. Meanwhile, toast2 slices whole-grain bread.
2. Top toasts with 1/4 avocado, sliced; a few slices of red onion;and muenster cheese. You have constructed two open-face sandwiches.
3. Put sandwiches in the oven just long enough to melt the cheese. (I would use a toaster oven if I had one).
I first discovered this type of sandwich at the ABC Cafe in Ithaca, where it was made with a bagel instead of bread, and topped just before serving with a handful of alfalfa sprouts.