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 pan or skillet. Grill or pan-fry 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. Pile onion rings and a dollop of dipping sauce along side. For drinks, try black and white frozen shakes -- yum!