In a large pot, bring water to a boil and add the cubed sweet potatoes. Cook for about 15 minutes or until tender. Drain and let cool slightly.
In a large mixing bowl, mash the sweet potatoes with a fork or potato masher until smooth.
Add the black beans to the bowl and mash them slightly, leaving some whole for texture.
Stir in the cooked quinoa, breadcrumbs, red onion, garlic, cumin, chili powder, salt, black pepper, lime juice, and cilantro (if using). Mix until well combined.
Form the mixture into patties, about 1/2 inch thick. You should get around 8 patties.
Heat a skillet over medium heat and add a thin layer of oil. Once hot, add the patties in batches, cooking for about 4-5 minutes on each side or until golden brown.
Remove the patties from the skillet and place them on a paper towel-lined plate to absorb excess oil.
Serve warm with additional lime wedges and your favorite dipping sauce or toppings.