Preheat oven to 400°F. Rub sweet potatoes with olive oil, poke a few holes with a fork, and place on a foil-lined baking sheet. Roast for 35–45 minutes, until a knife slides in easily.
While potatoes roast, heat a large skillet over medium heat. Add ground turkey and onion, breaking up the meat with a wooden spoon. Cook 6–8 minutes, until no pink remains.
Stir in garlic, chili powder, cumin, smoked paprika, oregano, and salt. Cook 1 minute, until fragrant.
Pour in water or salsa, scrape up any browned bits, and simmer 3–4 minutes until the mixture is saucy but not wet. Taste and adjust salt.
Split each roasted sweet potato open lengthwise and fluff the inside with a fork. Spoon the turkey mixture evenly into the potatoes.
Top with cheese, lettuce, avocado, and a generous dollop of Greek yogurt. Sprinkle with cilantro if using, and serve hot.