Combine egg, soft bread crumbs, and onion.
Add ham; mix well.
Form into four patties slightly larger than pineapple rings.
Make a hole in the center of each patty.
In 12 x 7 1/2 x 2-inch baking dish place ham patties atop pineapple rings.
Micro-cook, covered, till meat is done, about 6 minutes, giving dish a half turn and turning patties over once.
Spread buns with butter and mustard.
Place a ham patty and pineapple ring on bottom half of each bun; top with lettuce leaf, then bun top.