1. Preheat oven to 375°F. Line a large, rimmed baking sheet with foil; grease lightly.
2. Warm oil in a large skillet over medium-high heat. Add celery and onion and cook, stirring often, until softened, 3 to 5 minutes. Add garlic and sauté 1 minute longer.
3. Pour beans into a large bowl and use a fork or potato masher to mash into a thick paste. Scrape vegetables from skillet into bowl. Stir in egg, cumin and bread crumbs. Season with salt and pepper. Use your fingers to form into 4 patties (do not overmix). Place patties on baking sheet and bake until firm and set, about 10 minutes on each side. Serve on whole-grain buns with lettuce, tomato and sliced red onion, if desired.