• Preheat oven to 400 degrees F. Line a large rimmed baking sheet with parchment paper and set aside.
• Scrub the sweet potatoes until clean and dry them with paper towels. Place on the prepared baking sheet and poke with a fork about 3 or 4 times per side. Drizzle each sweet potato with a little olive oil, if using. Use your hands to rub the oil and coat all sides of each sweet potato with the oil. Bake for 50 (small) to 60 (medium) minutes, or until tender all the way through when pierced with a toothpick (don't use anything large like a knife to test them as it will allow them to start releasing more liquid).
• While the sweet potatoes are cooking, proceed with the rest of the recipe.

For the Spice Mix

• Combine the spices in a small bowl. Set aside.

For the Sauce

• Combine the yogurt with 1 teaspoon of the spice mix (reserve remaining spice mix for the filling), juice from half of the lime juice and 1 teaspoon lime zest. Mix to combine well and refrigerate while you make the taco filling.

For the Taco Filling

• While the sweet potatoes bake, coat a 12-inch nonstick skillet with nonstick cooking spray and place it over MEDIUM heat. When hot, add the ground turkey and onion and cook, breaking up the meat with a wooden spoon, 5 minutes. Drain any excess grease and return the skillet to the heat.
• Stir in the tomato paste, mixing it into the meat until well combined. Add the remaining spice mix and cook, stirring for a minute or two, or until the meat is well coated with the spices. Add the water and reduce the heat under the skillet to MEDIUM-LOW. Simmer for about 5 minutes or until the sauce has slightly reduced. Add the black beans and corn and cook, stirring occasionally until everything is warmed through. Squeeze the juice from the remaining lime half over the mixture, stir and remove from the heat.

Assemble

• Remove the sweet potatoes from the oven and allow them to rest for about 5 minutes. Then, slice each one in half.
• Place half a sweet potato in a bowl, top with the taco filling any optional toppings you’d like and a dollop of the sauce.