Mix all of the ingredients for the sauce into a bowl. Adjust any of the ingredients as needed to taste. Add a drop of water about a 1/4 teaspoon at a time as needed to thin the yogurt to a creamy sauce. Season with salt and pepper to taste. Set aisde.
In a bowl mix the beef, garlic powder, cumin, salt, and pepper, and half of the cilantro until fully combined.
Heat a nonstick pan on medium heat. Once hot, add the oil. Saute the onion until soft. Brown the beef mixture while breaking it up into smaller pieces until it is no longer pink and cooked through. Season with salt and pepper to taste as needed.
Warm the taco shells on both sides in a pan over medium heat for approximately 10 seconds per side.
Layer the taco shells with beef, lettuce, tomato, and feta. Drizzle with sauce.