Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a large skillet, heat olive oil over medium heat. Add diced onion and cook until translucent, about 3-4 minutes.
Add minced garlic and ground turkey to the skillet. Cook until the turkey is browned and cooked through, breaking it apart with a spatula, about 5-7 minutes.
Stir in chili powder, cumin, paprika, salt, black pepper, and cayenne pepper (if using). Cook for an additional 2 minutes to combine flavors.
While the turkey mixture is cooking, wrap the corn tortillas in aluminum foil and place them in the oven for about 10 minutes to warm.
To prepare the avocado sauce, combine avocado, Greek yogurt, lime juice, garlic powder, salt, and pepper in a blender or food processor. Blend until smooth and creamy.
Once the tortillas are warmed, remove them from the oven. Fill each tortilla with the turkey mixture, then top with shredded lettuce, diced tomatoes, and shredded cheese.
Place the filled tacos on the prepared baking sheet and bake for an additional 5-7 minutes, until the tortillas are crispy and the cheese is melted.
Serve the tacos hot, drizzled with avocado sauce and a dollop of sour cream on the side.