Preheat your oven to 180℃ (350 ℉)
Peel and chop the sweet potatoes into 1/2 inch (1 cm) cubes. Drain and rinse the beans and lentils. Chop the bell pepper and onions roughly. Peel the garlic cloves.
Put the sweet potato cubes in an aluminum foil packet and add them to a baking sheet. Put the rest of the veggies on the baking sheet and drizzle the olive oil over them. Sprinkle the salt and pepper all over and close the aluminum foil packet of the sweet potatoes as tight as you can. Bake the veggies at 350℉ (180℃) for 40-45 minutes. You'll know they are done when the sweet potatoes, peppers and onions are soft and the beans and lentils appear dry.
Let the veggies cool on the pan for several minutes until you're able to handle them with bare hands.
Put the peppers, onions, garlic cloves, and about 2/3 of the beans, lentils and sweet potato into a food processor. Leave the remaining 1/3 of the beans, lentils and sweet potatoes set aside in a large bowl.
Process everything you've added to the food processor until it's almost smooth but still has some texture (not a smooth puree). Add it to the bowl along with the remaining beans, lentils and sweet potato.
Add in the mustard, Worcestershire sauce and all of the spices. Mix well with your hands just until combined and all the ingredients are distributed evenly throughout the mixture. Add the breadcrumbs little by little until you get a "dough like" consistency for your burger patties or until they resemble meat patties.
Refrigerate the mixture for about 30 minutes before forming the patties. It's possible to skip this step, but forming the patties will be a lot easier if the mixture has been refrigerated first.
After refrigerating, divide the mixture into 6 balls that are exactly the same size. Use a burger press to form the burgers into a patty shape or use your hands to pack the burgers into evenly shaped patties.
Grill over medium-high heat (or fry in a frying pan over medium-high heat) for about 5-6 minutes per side until the burgers have grill marks and are heated through.
Serve on toasted buns with lettuce, tomatoes, cheese, onions and our Easy Homemade Burger Sauce.