Heat the olive oil in a large skillet over medium-high heat.
Add the diced onion and ground turkey to the skillet and cook about 5 minutes, breaking the meat apart, until the turkey is browned and the onion is softened.
Stir in the minced garlic and cook 30 seconds until fragrant.
Add the tomato sauce, cumin, chili powder, dried oregano, smoked paprika, cayenne (if using), and salt; stir to combine.
Transfer the mixture to the slow cooker, cover, and cook on high for 2–3 hours until flavors meld.
Warm the corn tortillas, spoon the turkey mixture into each tortilla, and top with desired toppings before serving.
