In a food processor, blend half an onion, 5 cloves garlic, 1 cup packed cilantro leaves, and 2 tablespoons lime juice. Puree well.
Rinse and drain a can of black beans. Add to the food processor, along with cumin, chili powder, and salt. Pulse until just combined. Add flour and baking powder and pulse until incorporated.
Spread 1 tablespoon of oil over a large baking sheet. Take large spoonfuls of the bean mixture and form into disks - about 3 inches across and 1/2 an inch or so tall. Place them on the baking sheet. Once all the mixture has been used, brush the tops with the remaining 1 tablespoon of oil.
Bake for 25-30 minutes, flipping halfway. You want them crispy on the outside but still soft inside.
If desired, mix Greek yogurt, salsa, and lime juice for a white sauce to go with the falafel. Serve with garnishes of cheese, fresh squeezed lime juice, and avocado, or even wrap them up in a tortilla with lettuce and tomato.