Place the chicken stock in a sauce pan, add the tomato paste, cumin, chili powder, and oregano. Bring to a boil, then reduce the heat to medium and add the shredded chicken. Season with salt and pepper, cook for about 3 to 5 minutes.
Heat the tortillas on a nonstick skillet, then wrap them in aluminum foil to keep them warm.
To assemble: spread one tortilla with sour cream, then top with chicken, tomatillo salsa, avocado and queso fresco. Season with salt and pepper to taste and serve with lime on the side. Repeat the process with the remaining tortillas or just let every person assemble their own taco.