2 slices whole-wheat country bread, crusts removed, torn into pieces
1/3 cup fresh or frozen and thawed blueberries
1 tablespoon balsamic vinegar
2 teaspoons Dijon mustard
1 teaspoon Worcestershire sauce
2 cloves garlic, minced
1/4 teaspoon salt, or to taste
Freshly ground pepper, to taste
12 ounces 90%-lean ground beef
Place bread in a food processor and pulse into fine crumbs. Transfer to a large bowl. (No need to wash the workbowl.)
Add blueberries, vinegar, mustard, Worcestershire, garlic, salt and pepper to the food processor; process until pureed. Scrape into the bowl with the breadcrumbs. Add ground beef and mix well with a potato masher. Divide the mixture into four equal portions; form into 1/2-inch-thick patties, about 4 inches in diameter.
Meanwhile, preheat broiler or heat an indoor or outdoor grill to medium-high. If using the broiler, coat a broiler pan with cooking spray. If using a grill, oil the grill rack by rubbing it with an oil-soaked paper towel. Cook patties until browned and no longer pink in the center, 4 to 5 minutes per side. An instant-read thermometer inserted in the center should register 160°F. Serve immediately, with or without rolls and toppings.