In a skillet, heat olive oil over medium heat. Saute onion, garlic and pepper until softened. Add carrots and mushrooms and heat until desired tenderness; cool.
Preheat grill to medium heat.
Add cooled vegetables to meat mixture and combine. Form into 4 patties.
Coat grill with non-stick spray and cook patties for 4-6 minutes per side.
Serve on wheat or pretzel buns and top with lettuce.