Preheat the oven to 325 degrees F. Heat olive oil in a large skillet over medium heat. Add onions and sauté until starting to brown (about 10 minutes). Add the garlic and cook for another minute.
Add the chicken, paprika, chili powder, cumin, oregano, cayenne, and salt. Turn the heat to high and use a wooden spoon to stir and break up the chicken into small chunks until the chicken is partially cooked (about 5 minutes). Stir in tomato sauce, reduce heat to low and simmer, stirring occasionally for about 10-15 minutes or until the chicken is cooked through. Sprinkle with fresh cilantro.
Lay the taco shells on to a baking sheet. Place into the oven and bake for 6-7 minutes or until crispy. Spoon the chicken into the shells and serve with toppings.