PICKLED ONIONS: Thinly slice the onions (I used a mandolin) and place in a large mason jar. In a saucepan, heat apple cider vinegar, sugar, salt and peppercorns until dissolved. Pour cider mixture over the onions and cover with a lid. Let sit for at least an hour. Onions can be stored in refrigerator for up to two weeks.
AVOCADAO CREMA: In a food processor or blender, add avocado, crema, garlic, cumin, salt, pepper and cayenne; process until smooth and creamy.
BRISKET: Heat olive oil in a fry pan over medium heat. Add leftover brisket or pot roast, green chilies, garlic powder, cumin, cayenne pepper, oregano, salt and pepper. Stir to combine. Heat though until the meat mixture begins to brown. Remove from heat.
TO ASSEMBLE: Warm the corn tortillas in the microwave for 30 seconds. Place a small amount of the meat mixture in the tortilla. Top with avocado crema, cojita cheese, pickled onions, and cilantro. Serve and Enjoy!