Prepare special sauce by combining all ingredients in a bowl and stirring well, add water to achieve desired consistency, set aside.

Heat olive oil in a medium sauté pan over medium high heat. Add onions and mushrooms and sauté 10 to 12 minutes or until caramelized and soft. Transfer to a large bowl and cool slightly.

To the bowl, add ground beef and freshly cracked black pepper. Divide into 4 portions and form into ½-inch thick patties.

Clean the saute pan and add a little more oil just to coat the bottom. Cook over medium-low heat for 10 minutes, turning half way through or cook burgers on a preheated grill for 8 minutes per side for medium rare or about 12 minutes per side for medium or longer depending on desired doneness.

Remove from grill or pan and let rest for 5 minutes. Place each burger on a whole grain bun with 1 lettuce leaf, 1 tablespoon Special Sauce, and a slice of tomato.