12 ouces 93%-lean ground beef
1/4 teaspoon freshly ground pepper
2 cups (about 2) finely chopped portobello mushroom caps, gills removed
4 slices rye bread
3 tablespoons plain dry breadcrumbs
1 clove garlic, peeled
1 tablespoon Worcestershire sauce
8 teaspoons pickle relish
1 teaspoon chopped fresh thyme, or 1/2 teaspoon dried
4 (2 ounces) slices Swiss cheese
1/4 teaspoon salt
Position rack in upper third of oven; preheat broiler. Line a broiler pan with foil.
Gently mix beef, mushrooms, breadcrumbs, Worcestershire, thyme, salt and pepper in a medium bowl until combined. Shape into 4 patties and place on the prepared pan. Broil until cooked though, 4 to 6 minutes per side.
Meanwhile, toast bread. Rub each slice of toast with garlic.
Place 1 patty on each piece of toast, top each with 2 teaspoons relish and cover with a slice of cheese. Remove foil and place the sandwiches on the pan; broil until the cheese is just melted, 30 to 60 seconds.