Preheat outdoor grill to MEDIUM-HIGH.
Form beef into 4 patties; grill to desired level of doneness. While burgers are cooking, heat the olive oil in a large skillet over medium heat.
Add onion, mushrooms and bell pepper; sauté until onion is translucent. Evenly top each cooked burger with vegetable mixture and a slice of cheese.
LC SERVING SUGGESTION: A big bowl of coleslaw (use a ready-made mix and toss with low fat mayo and a little rice vinegar).