1 small onion, cut into quarters
1/8 teaspoon white vinegar
1/2 of an 8-ounce package button mushrooms, cleaned and trimmed
Salt and pepper, to taste
1 pound lean ground beef
6 slices sharp cheddar cheese
1/4 teaspoon salt
18 pickle rounds
2 tablespoons 0% Greek yogurt
6 lettuce leaves
1 tablespoon ketchup
6 whole-wheat buns
1 teaspoon sweet relish
Using a food processor or a knife, finely chop the onions and mushrooms. In a large bowl, combine the ground beef, onion and mushrooms. Season with the salt and mix well. Divide the mixture into 12 portions and roll each into a ball.
Flatten each ball between the palms of your hands. Set patties on a baking sheet and refrigerate while you make the sauce.
In a small bowl, combine the yogurt, ketchup, relish, vinegar, salt and pepper; set aside.
In a large nonstick skillet coated with cooking spray, cook the burgers over medium-high heat, in batches, about 3 minutes per side, or until cooked to your desired doneness. Top 6 of the patties with cheese slices as soon as they come out of the pan.
Assemble the burgers: Layer 2 patties (one of them with cheese), 3 pickle slices, a piece of lettuce and some of the sauce on each bun.