In a small saucepan, combine the rinsed quinoa with 3/4 cup water and bring to a boil. Reduce to a simmer and cook for about 15 minutes. At this point most of the water should be absorbed. Cover the pot with a lid and allow the quinoa to steam and absorb any remaining water.
Meanwhile, in a large skillet, warm 1 tablespoon oil over medium heat. Add the minced onion, pepper and garlic then cook until onion is translucent, about 5 minutes. Remove from heat and set aside to cool.
In a large bowl, add the black beans and mash with a fork. Add the cooked quinoa and onion mixture, as well as the cumin, chili powder and salt. Stir to combine. Add the flax egg and stir a few more times.
Sprinkle half of the oat flour into the bowl and stir to combine. Add the remaining oat flour and continue to mix until evenly combined.
Using your hands, form the batter into 5 separate patties (about 1/4 cup of batter at a time). Warm the remaining two tablespoons oil in the large skillet over medium heat. Add the burger patties and cook for about 7 minutes on each side, until light golden brown.
Serve warm on toasted buns with desired toppings and enjoy!