1. If using mature spinach, trim off any tough stems.
2. Heat 1 tablespoon of the oil in a medium skillet over medium heat. Add the spinach and 1 pinch of salt, and cook, turning the spinach with tongs, until wilted but still bright green, 1 to 2 minutes.
3. Gather the greens to one side of the pan and, holding the pan over the sink, press against the greens with a spatula and pour off the excess liquid. (You can do this in a colander if you’re worried about accidentally dumping the greens into the sink.) Transfer the greens to a plate or bowl, wipe out the skillet, and return it to the heat.
4. Raise the heat slightly and add the remaining 3 tablespoons of the oil to the skillet. Once hot, add the onion. Cook, stirring frequently, for 8 to 15 minutes, until reddish-golden brown and beginning to crisp. Use a slotted spoon to transfer the onions to a paper-towel-lined plate to drain, leaving the oil in the skillet. Pour off all but about 1½ tablespoons of the oil, and return the skillet to the heat.
5. Slice the sweet potato in half lengthwise and then into quarters. Slice each quarter into ¼-inch-thick pieces. Add the cayenne to the hot oil, followed by the sweet potato. Sprinkle with another big pinch of salt and spread the sweet potato into as even a layer as space allows. Cook for 2 to 3 minutes without disturbing, until potato takes on some color. Flip and repeat. Continue cooking until the sweet potato is light golden brown and tender, 4 to 7 minutes total (check by piercing with a paring knife).
6. Drizzle the maple syrup over the sweet potato, and toss to coat. Cook for another 1 to 2 minutes, until the maple syrup thickens and caramelizes, glazing the sweet potatoes. Use a slotted spoon to transfer the sweet potato to the plate with the spinach, leaving the maple syrup and oil in the skillet. Return the skillet to the heat.
7. Add the rice to the skillet, breaking it up with a spatula. Cook for 3 to 5 minutes, stirring constantly, until the rice is heated through and the pan appears dry.
8. Divide the hot rice between two bowls, then divide the spinach and potato over the rice. Top each serving with 1 fried egg, and garnish with the fried onions. Dollop with 1 tablespoon of the yogurt, top with a few grinds of black pepper, and serve, passing hot sauce at the table.