Swiss Mushroom Burger

• 1 small red onion
• 1 lb. (450 g) extra-lean ground beef
• 1 egg
• 1 Tbsp. finely chopped fresh parsley
• 2 cloves garlic, minced
• 2 Tbsp. Dijon mustard, divided
• 1 large portobello mushroom, stem and gills removed
• 2 Tbsp. Aged Balsamic Vinaigrette Dressing
• 1 cup shredded Swiss cheese
• 1/4 cup light mayonnaise
• 4 hamburger buns

To Make:

1. Heat barbecue to medium heat.

2. Cut onion crosswise in half. Cut half the onion into 1/4-inch-thick slices; finely chop remaining onion. Mix meat, chopped onions, egg, parsley, garlic and 1 Tbsp. mustard just until blended; shape into 4 (1/2-inch-thick) patties.

3. Brush mushroom and onion slices with dressing. Grill patties and vegetables 6 min., turning vegetables after 3 min. Remove vegetables from barbecue. Turn patties; grill 5 min. (Burgers will not be done.) Meanwhile, cut mushroom into thin slices.

4. Top burgers with mushrooms, cheese and onions; grill 3 to 4 min. or until burgers are done (160ºF) and cheese is melted. Mix remaining mustard and mayonnaise; spread onto buns. Fill with burgers