In a medium bowl, mix all ingredients with hands until distributed evenly. Form mixture into 4 patties.
Place on oiled grill pan or grill and cook 5 to 7 minutes on each side or until internal temperature is 165°F.
Serve on buns with sliced onion, lettuce leaves, and rhubarb ketchup.
Note
To make about 3/4 cup panko-style bread crumbs, process 3 slices of bread into coarse crumbs. Place crumbs on a baking sheet and bake at 300°F, shaking sheet once or twice, for 6 to 8 minutes or until crumbs are dry but not browned. Store any extra crumbs in the freezer in airtight container.