Add oil and diced onions in a large skillet. Over medium-high heat, sauté onions until translucent, occasionally stirring for 3-4 minutes. Once cooked, transfer to a bowl and aside to cool.
Add the drained black beans, corn, boiled sweet potatoes, sautéed onions, spices, lemon juice, and cilantro in a food processor.
Pulse until completely incorporated.
Mix in cooked quinoa.
Add oats flour and mix until combined. The mixture should feel moist, not dry.
Grease your hands with oil. Greasing will prevent the burger mix from sticking to your hands. Refrigerate the patties for about 10 minutes before cooking. Chilled patties will hold better shape while cooking and will not disintegrate in oil.
Heat about 2 tablespoons of olive oil in a pan over medium heat. Add the black bean and quinoa burgers to it. Be careful not to overcrowd the pan.
Once the burgers start to sizzle, lower the heat and cook the burgers for about 4-6 minutes on each side, until crispy and cooked through. Once cooked, remove from heat.
Assemble: the patties with your favorite toppings, buns, or serve them as a salad topper.