Beef Stroganoff Burgers

Name: Beef Stroganoff Burgers
Ingredients: 5 portobello mushrooms (4 to 5 ounces each); 2 tablespoons plus 2 teaspoons olive oil; Kosher salt and freshly ground black pepper; 1 small onion, diced; 1 medium garlic clove, minced; 1 teaspoon all-purpose flour; 1/2 cup low-sodium beef broth; 1/4 cup reduced-fat sour cream; 3 tablespoons chopped fresh flat-leaf parsley; 1 tablespoon Dijon mustard; 2 teaspoons Worcestershire sauce; 1 pound ground beef (95 percent lean); Nonstick cooking spray; 4 hamburger buns; Lettuce leaves for topping