In a large mixing bowl, mash the rinsed black beans using a fork until mostly smooth, leaving some chunks for texture.

Add the cooked quinoa, bread crumbs, chopped onion, minced garlic, cumin, smoked paprika, salt, black pepper, and egg to the bowl.

Mix everything together until well combined. The mixture should hold together but not be too wet. If it's too dry, add a little water; if too wet, add more bread crumbs.

Divide the mixture into four equal portions and shape each portion into a patty, about 1/2 inch thick.

Place the patties on a plate and refrigerate for at least 15 minutes to help them firm up, which will make them easier to cook.

Heat olive oil in a skillet over medium heat. Once hot, carefully add the patties to the skillet.

Cook for about 5-7 minutes on each side, or until they are golden brown and heated through. Avoid flipping them too early to prevent breaking.

Toast the burger buns lightly on the skillet or in a toaster.

Place a lettuce leaf on the bottom half of each bun, followed by a cooked black bean patty, tomato slices, avocado, and salsa if desired.

Top with the other half of the bun and serve immediately.