In a small skillet heat the oil over moderately high heat until it is hot but not smoking, add the tortillas, 1 at a time, for 5 seconds, or until they are softened but not crisp, and transfer them with tongs, letting the excess oil drip off, to paper-towel-lined baking sheets to drain.
Preheat the oven to 350 degrees F.
On each tortilla mound 1/4 cup of the turkey, 1 tablespoon of the Cheddar, 1 tablespoon of the onion, and 1 teaspoon of the chopped coriander, roll up the tortillas jelly-roll fashion, and arrange the enchiladas, seams side down, in a shallow 3-quart baking dish. Pour the chili sauce over the enchiladas, sprinkle the enchiladas with the remaining 2 1/4 cups Cheddar, and bake them in the oven for 15 to 20 minutes, or until the cheese is bubbly. Sprinkle the enchiladas with the remaining chopped coriander. Serve the enchiladas on heated plates, garnished with the avocado, peeled and diced, and the coriander sprigs.