Place beans in a large mixing bowl. Use a potato masher to crush them until they're mostly mashed but some whole beans still remain (you want about 3/4 of the beans to be fully mashed). Alternatively, you can use a large fork to do this, but it will take much longer.
Once the beans are (mostly) mashed, add the breadcrumbs, grated onion, egg, chili powder, salt, pepper, and Worcester sauce to the bowl. Stir until everyting is mixed together well.
Form the mixture into 6-8 patties, depending on how large you want them to be. This is a great time to freeze some of the burgers to cook at a later time.
Heat the butter and olive oil in a large skillet over medium heat. Once the butter/oil mixture is hot, place the patties in the skillet and cook for 5 minutes. Flip the paties and cook for another 2 minutes, or until both sides are seared.
Serve the burgers over leaves of butter lettuce with sliced avocado, mustard, ketchup, and your favorite burger toppings. These would also be great on burger buns!