Preheat oven to 350 degrees. Wrap the tortillas in foil and heat in 350 degree oven for fifteen minutes.
Increase the heat under the skillet to medium-high and cook until the liquid is reduced to 1/4 cup, about 5 minutes. Remove skillet from the heat and whisk in the mustard. Set the skillet aside.
Serve with tortillas and desired taco toppings.