Preheat grill to medium heat. Grease grill with 1 tablespoon olive oil using a paper towel or pastry brush.
Place the mushrooms on a cutting board, and carefully remove mushroom stems with a paring knife. Scrape out the inside portion, and place the mushrooms into a food processor and pulse until pureed. Place the pureed mushrooms onto a paper towel to remove any excess moisture, and set aside.
In a medium size cooking skillet over medium-high heat, add 2 teaspoons olive oil, onions, and garlic. Sauté until soft.
Add mushrooms and sauté for 5 minutes, or until golden brown. Remove from heat and place into a medium size mixing bowl to cool, about 5 minutes.
Mix in ground beef, Dijon mustard, and salt to the mushroom mixture, and combine with a spatula.
Divide evenly into 5 lightly packed balls. Using clean hands, gently flatten the balls into patties approximately ¾ inch thick and 4 inches in diameter.
Place burgers on a preheated grill and cook burgers over medium heat for approximately 5 minutes on each side, or until the internal temperature is 160°f or desired doneness. Serve with desired toppings.
*Sautéing mushrooms before adding them to the ground meat enhances the flavor of the mushrooms, however, if you are short on time, it is not a necessary step.
Serving size: 1 hamburger Calories: 265 calories, Fat: 20 grams, Saturated fat: 7 grams, Carbohydrates: 3 grams, Sugar: 1 gram, Sodium: 255 milligrams, Fiber: 0 grams, Protein: 16 grams, Cholesterol: 65 milligrams