Preheat the oven to 350°F (175°C). Wrap the tortillas in foil and warm them in the oven for 10 minutes until softened, or microwave on a plate covered with a damp paper towel for 20 seconds. Set aside. In a 1-quart saucepan, melt the butter over medium-high heat. Add the chopped onion, garlic, coriander, cumin, salt, and black pepper. Cook for about 2 minutes, stirring occasionally, until the onion is soft. In a small bowl, stir the flour into the sour cream until smooth, then add it to the pan.