Sirloin Burgers with Balsamic Mayo, Mushrooms and Swiss

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1/4 cup chopped onion

1 1/3 pounds ground sirloin, 90 percent lean

1 tablespoon Worcestershire sauce, eyeball the amount

Steak Seasoning Blend or, coarse salt and pepper, preferred brand McCormick Montreal Steak Seasoning

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, eyeball the amount

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