1. Preheat the oven to 375° F. Poke the sweet potato all over with a fork and wrap it in aluminum foil. Place on a baking sheet and roast until tender, about 1 hour. Let cool for 10 minutes and roughly chop. Makes 1 cup of cooked sweet potato.
2. In a large skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the onions, season with a pinch of salt, and sauté until soft and golden, 5 minutes. Add the garlic and cook, stirring often, until the onions and garlic are golden browns, 5 minutes more. Remove from the heat and transfer to a food processor. Wipe the skillet clean and reserve for later use.
3. To the food processor, add the chopped sweet potato, quinoa, black beans, panko bread crumbs, ½ teaspoon of salt, pepper, paprika, and cumin, and pulse until the mixture is slightly chunky. Use your hands to form the mixture into 6 patties and transfer to a large plate. Chill in the fridge to firm up, uncovered, for 30 minutes or up to 1 day.
4. In a large skillet, heat 1½ teaspoon of olive oil over medium-high heat. Working in 2 batches, add the patties and cook until both sides are golden brown, 4 to 6 minutes per side. Remove the patties and drain them on a paper towel-lined plate.
5. Spread the mayo on each bottom bun half. Divide the patties among the buns and top each with spinach, tomato, onion, and a generous spoonful of guacamole. Top with the other bun halves and serve.