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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.