To make the pickled onions, pack the red onions in a large mason jar. Pour the water, vinegar and salt on top and shake. If it needs more liquid, just add equal parts water and vinegar. Press onions down so they are all covered. Let it sit overnight or for at least 1 hour. (See How-To Video!).
Preheat the oven to 400F. Line a baking sheet with parchment or tin foil. Wash your sweet potatoes, then prick them all over with a fork or knife. Bake for 45-60 minutes, or until a knife inserts easily without resistance. Let cool.
In a large skillet over medium heat, add the olive oil, onion, and ¼ teaspoon salt. Cook until softened, about 3-5 minutes. Add the tomato paste, garlic, chili powder or cumin, and red pepper flakes (if using). Cook, stirring constantly, for about 30 seconds. Add the spinach and ½ teaspoon salt and cook for 2 more minutes, or until the spinach has wilted, stirring often. Stir in the black beans and remaining ¾ teaspoon salt and cook for 3 more minutes, stirring occasionally. Remove from heat and stir in the lime juice. Taste and add more salt or lime, if needed.
Transfer to a bowl or container, wipe out the skillet, and fry your eggs.
Cut an “X” into your warm baked sweet potatoes (see photos) and pry the potato open. If using 2 large potatoes, just cut in half lengthwise. Use a fork to cut/mash up the flesh and sprinkle with a pinch of salt. Top each sweet potato with the warm bean mixture, fried egg, and pickled red onions (I like them chopped up for this recipe, but you can leave them whole). Sprinkle the egg with salt and top everything with a squeeze of lime and hot sauce (if using).