Heat the oven to 250. Place the tortillas, one at a time, in a large nonstick skillet over medium heat and warm for 1 minute, flipping once or twice, until soft. Wrap them in a dish towel and keep them warm in the oven.
To make the beef filling, heat the oil in the skillet over medium. Add the onion and saute for 3 minutes or until soft.
Add the ground beef and cook, stirring to break up the meat, until it's no longer pink (about 4 minutes). Pour off the liquid.
Add the garlic, chili powder, cumin, coriander, salt, and tomato sauce. Cook, stirring occasionally, over low heat for 10 minutes. (If the sauce is too thick, add a few tablespoons of water.)
Meanwhile, place your toppings in separate small bowls and set them on the table. Choose four or more toppings, depending on your family's tastes.
Place the warm tortillas in a basket and the beef filling in a bowl. Allow each person to layer ingredients into his or her own tortilla, then fold them up.