For a super-healthy take on crema, blend avocado with thick and creamy nonfat Greek yogurt. Slideshow: More Taco Recipes
In a medium bowl, toss the tomatoes with half of the cilantro and the juice of 1 lime; season with salt. Let stand for 10 minutes, stirring occasionally.
Meanwhile, in a small food processor, puree the avocado with the yogurt, garlic, 1/2 teaspoon salt and the remaining lime juice.
In a large skillet, heat the olive oil. In a medium bowl, toss the chicken with the chile powder and cumin and season with salt. Add the chicken to the skillet and cook over high heat, turning occasionally, until nicely browned on all sides, about 8 minutes total.
On a microwave-safe plate, microwave the tortillas at high power for 30 seconds, until soft and pliable. Spread the crema on the tortillas, top with the chicken and tomatoes and serve.