Preheat the oven to 350ºF. Spread out the black beans on a large rimmed baking sheet so that they are in a single layer. Place them in the oven to dry out until they start to split, about 15 minutes.
Meanwhile, heat the olive oil in a skillet over medium high heat. Saute the onion until softened, about 5 minutes, then add in the garlic and stir for one more minute. Remove from the heat.
In a large bowl, combine the sauteed onion and garlic, beans, sweet potato, oat flour (or flax), BBQ sauce, cumin and salt. Use a fork to stir the mixture, mashing the beans to help the batter stick together. You don't want to mash all of the beans, as they help add texture, but you want it mashed enough that you can form and shape the burger patties with your hands.
Use a 1/3 cup measure to scoop out the black bean mixture and use your hands to shape it into a burger about 3/4-inch thick. Line a baking sheet with parchment paper (I use the same one I used for drying the beans) and arrange the burger patties on the pan as you go. Use wet hands to shape the patties to help prevent sticking, if necessary. This recipe should make 8 to 9 burgers.
Bake the burgers at 350ºF for 15 minutes, then use a spatula to gently flip them over and bake for another 5 to 10 minutes. The finished burgers will feel a little dry around the edges, but still slightly soft to the touch in the center. As they cool, they will firm up and dry out a bit more, so you don't want to over cook them.
Serve the burgers warm with your favorite toppings. Leftovers can be stored in an airtight container in the fridge for up to a week, or in the freezer for up to 3 months.