Bake sweet potato in 400 degree oven for 40-60 minutes, or until tender. Alternatively, cook in microwave to save time. Allow to cool slightly; then peel.
Add sweet potato and beans to a large mixing bowl. Using a large fork, mash well. Add the chopped onion, continuing to mash. Add in the remaining burger ingredients and mash well until thickened and thoroughly combined.
Heat oven to 400 degrees.
Heat a skillet over high heat and add the olive oil.
Form burger mixture into 6 large patties and place on the hot skillet, in batches if necessary. If using Panko bread crumbs, first roll patties in Panko to coat well. Cook 1-3 minutes on each side, to brown. Tip: Chill mixture in the fridge for a half hour to more easily form into patties.
Place the seared patties on a baking sheet and bake for 10-15 minutes, until cooked through.
Toast buns, if desired. Top with mayonnaise, sweet potato patty, sliced avocado, tomato, and spinach. Serve warm.
Recipe adapted from: kblog.lunchboxbunch.com