2-1/4 lbs. ground buffalo (or you can also use ground beef) Olive oil Salt and freshly ground black pepper 6 thick slices sharp Cheddar cheese 6 tomato slices 2 cups gourmet mixed greens
Heat the butter and olive oil in a large skillet over medium-high heat. Add the onions and saute for about 5 minutes. Reduce heat to medium low and continue to cook the onions, turning occasionally, until softened and ligthly browned in spots, about 10 minutes. Add the balsamic vinegar, stir to coat the onions evenly; cook for another 5 minutes. Season to taste with salt and freshly ground black pepper. Remove from heat and set aside.
Mix the ketchup with the chipotle powder and balsamic vinegar in a medium bowl. Adjust seasonings to taste with salt and freshly ground black pepper. Set aside.
Prepare a grill for medium high heat. Shape the buffalo meat into 6 round patties. Lightly rub each side of the patties with olive oil and season both sides with salt and freshly ground black pepper.
Place burgers on the grill. Close the cover and cook the burgers until the bottoms start to darken and juices rise to the surface, about 3 minutes. Turn the burgers; cook to the desired doneness, about 3-4 minutes longer for medium rare. Top the caramelized onions and cheddar cheese. Close cover; cook just until the cheese melts.
Place the burgers on plates; spread with the chipotle ketchup. Top with tomatoes and gourmet greens. Serve.