Beef Barbacoa: Cut beef brisket into 1/4 inch slices in 1/2 inch strips. In a bowl, combine beef stock, barbeque lime juice, cumin, oregano and cloves. Mix well. Add beef mixture, onions and garlic to sauce mixture. Combine to evenly coat meat. Place mixture in oven proof pan and cover with foil. Place in a preheated 350 degree oven for 12 to 15 minutes.
Pickled Red Onions: Mix all ingredients together. Refrigerate to marinate for 4 hours.
For Service: Warm tortillas, place 2 tablespoon of beef in the center of each tortilla. Place 1 tablespoon of red onion on top of beef, place 1 tablespoon of cheese on top of onions, 1 tablespoon cilantro on top of cheese. Serve immediately.