We love bison as a lean, flavorful alternative to beef. For this burger, we combine smokey chipotles and warming spices in the patties. The gentle kick is tamed by a cooling avocado mash, and the crunch of crispy lettuce and white onion. It results in a hearty, full-flavored burger that's lower in calories than you might think!
Halve and pit the avocado, then scoop the flesh into a small bowl. Add the lime juice, cilantro, 1/4 teaspoon cumin and a big pinch of salt. Mash and stir until well combined; set aside.
Combine the bison, chipotles, remaining 3/4 teaspoon cumin, the oregano, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl; mix well. Form into 4 patties, about 4 1/2 inches wide and 1/2 to 3/4 inch thick. Heat the vegetable oil in a large nonstick skillet over medium-high heat. Add the patties and cook until browned, about 3 minutes per side for medium, putting 1 cheese slice on each burger during the last minute of cooking.
Serve the burgers on the buns with ketchup, the avocado mixture, shredded lettuce, sliced onion and cilantro. Serve with tortilla chips and the lime wedges.