- In a skillet, brown the mushrooms and shallot in the oil. Add the garlic and cook 1 minute.
- In a food processor, purée the mushrooms and lentils until grainy. Add the egg and breadcrumbs and pulse for a few seconds at a time until just smooth. Season with salt and pepper. Shape into four patties. If necessary, add breadcrumbs.
- Preheat the grill, setting the burners on medium. Oil the grate.
- Oil the mushrooms, red bell pepper, and lentil patties. Grill the vegetables. Season with salt and pepper. Grill the patties, for 3 to 4 minutes on each side, and the buns.
- Spread mayonnaise on the buns. Layer lettuce and patties on the bottom buns. Top with mushrooms, bell peppers, and tomatoes. Cover with the top buns. Serve immediately.