Mushroom & Swiss Sirloin Burgers w/Balsamic Mayo

This is a Rachael Ray recipe.

1/4 cup chopped onion
1 1/3 pounds ground sirloin
1 tablespoon Worcestershire sauce
Steak Seasoning Blend or, coarse salt and pepper
Extra-virgin olive oil, for drizzling plus 1 tablespoon, 1 turn of the pan in a slow stream
4 crusty rolls, split
4 portobello mushrooms, thinly sliced
Coarse salt and pepper
4 slices Swiss cheese
3 tablespoons aged Balsamic vinegar
1/2 cup mayonnaise or reduced fat mayonnaise
Coarse black pepper
1/4 pound baby spinach
2 vine ripe tomatoes, sliced
2 scallions, chopped
4 romaine leaves, optional

Preheat grill pan or large nonstick skillet. Saute onions in a small pan over moderate heat for 3 to 5 minutes until tender. Combine meat with Worcestershire sauce, Sauteed onion and steak seasoning blend or salt and pepper. Form 4 large patties. Drizzle patties with a touch of extra-virgin olive oil. Quick toast your split rolls, then add burgers to grill. Grill burgers 4 to 5 minutes on each side for medium well doneness.

Heat a small to medium nonstick skillet over medium high heat. Add a little oil and mushrooms. Season mushrooms with a little salt and pepper and saute until just tender, about 3 to 5 minutes. Remove from heat. Pile mushrooms on top of burgers just before you are ready to take them off the grill or out of the pan. Fold each Swiss cheese slice in half and rest on top of mushrooms. Place a loose tin foil tent over the burgers and turn heat off under pan or grill. Allow the cheese to begin to melt down over mushroom and burgers, about 2 to 3 minutes.

Combine balsamic, mayonnaise and coarse cracked black pepper in a small dish. To assemble, place burgers with mushrooms and Swiss cheese on bun bottom. Slather tops of buns with Balsamic black pepper mayonnaise and baby spinach. Set burger tops in place and serve with sliced vine ripe tomatoes, drizzled with oil, seasoned with salt and pepper and garnished with scallions.