INGREDIENTS
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8 (1/8-inch-thick) slices Vidalia or other sweet onion
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1 tablespoon balsamic vinegar
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Cooking spray
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1 pound extralean ground beef
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1/4 teaspoon salt
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1/4 teaspoon freshly ground black pepper
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3 tablespoons creamy mustard blend (such as Dijonnaise)
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8 (1-ounce) slices rye bread
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1 cup (4 ounces) shredded reduced-fat Jarlsberg cheese