On a sheet tray or baking pan, place onion, tomatillos, tomatoes, and garlic and drizzle with olive oil. Place under the broiler or roast in a preheated 500 degree F oven until the vegetables start to get a little charred on the outside. Remove and set aside. On another sheet tray or baking pan, place the dried chiles, and toast in a 500 degree F oven for 1 minute or until the chiles start to release some smoke. Remove and plunge in to bowl with hot water and set aside.
In a blender, begin pureeing the chiles and roasted vegetables in batches with chicken stock until smooth. Season with salt and pepper and set aside.
In a saute pan, toast the cumin, coriander, and mustard for 3 minutes or until they start to smoke. Remove from pan and place in a small mixing bowl. Add the olive oil, salt, and pepper, to make a paste. Rub this paste all over the piece of flank steak and allow to marinate for 30 minutes.
Preheat the oven to 350 degrees F. Preheat a grill or grill pan on high.
Grill the flank steak for 3 minutes on each side. Place the flank steak in a roasting pan and pour the pureed chile sauce over it. Add 2 cups of water, cover with foil, and cook in the oven for 1 hour until the meat becomes flaky.
Remove the meat from the pan, shred the meat, and reserve the sauce. Place 6 of the tortillas onto a baking sheet. Put a small amount of the shredded meat on the tortilla and spoon over some of the sauce. Top with some cheese. Repeat with the remaining ingredients. Bake for 5 minutes and serve.