Preheat your oven to 400 degrees. Spray a baking sheet with nonstick spray. Scrub your sweet potatoes clean and lay them on the baking sheet. Roast in the oven until soft, somewhere in the range of 1 hour. Actual baking time will depend on thickness and size of your sweet potatoes. Start checking them after 40 minutes if they are on the small side.
While your sweet potatoes are cooking, heat a small skillet to medium-high heat. Add in olive oil and then saute the onion in it for about 7 minutes, until the onions are soft and translucent. Add in the minced garlic, cumin, and chili powder and saute for another minute. Remove from heat and set aside.
Remove the sweet potatoes from the oven and carefully slice them in half. Scoop out the flesh and put it in a medium-sized mixing bowl. Leave a little flesh on the potato skin to create a wall. Set the skins back onto the baking sheet.
Once the flesh has been removed, mash it all up in the mixing bowl. Add in the onion mixture, black beans, and fresh spinach and stir until well combined. Stuff each skin full with the seasoned mixture. Top with cheese. Put them back in the oven for another 10-15 minutes until cheese is bubbling and starting to brown.