Sirloin Burgers with Balsamic Mayo, Mushrooms and Swiss

Ingredients

• 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
• 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