For the steak: Place the steak in a large resealable plastic bag. Blend the orange juice, sour mix, olive oil, soy sauce, brown sugar, cumin, chipotle pepper, garlic and lime juice in a blender until coarsely chopped. Pour into the bag with the steak. Add the onion and cilantro and refrigerate for at least 2 hours and preferably overnight.
For the avocado crema: Combine the avocadoes with the sour cream, cilantro, lime juice, cumin, garlic, chili powder and salt and pepper to taste. Either hand blend or process the mixture in a blender for a creamier sauce.
For assembly: Prepare a grill for medium-high heat. Coat the Serrano peppers, limes, onions and bell peppers with the olive oil and place them on the grill. Char the onions and lime wedges, turning occasionally, until grill marks form. Cook the peppers, turning occasionally, until they blister. Place the peppers in a paper bag for 10 minutes to let them sweat. Pull off the pepper skins and remove the caps and seeds. Reserve the lime wedges for serving and coarsely chop the peppers and onions and set aside.
Take the steak out of the refrigerator at least 30 minutes prior to grilling to bring to room temperature. Place the steak on the grill and char both sides, turning as often as you desire, until the internal temperature registers approximately 115 degrees on an instant-read thermometer, about 10 minutes. (Discard the marinade.) Let the steak rest for 10 minutes before thinly slicing across the grain. Once thinly sliced, chop.
Warm a pizza stone on the grill. Place a tortilla on the stone and sprinkle a light coating of cheese on top. Place another tortilla on top (the cheese will act as a binder when grilling). Add more cheese on top of the second tortilla. Then add a layer of chopped peppers and onions, followed by the chopped skirt steak. Dust with the cotija cheese and drizzle the avocado creme sauce over the entire surface. Sprinkle on some fresh cilantro and garnish with a grilled lime wedge or two. Repeat with the remaining 2 tortillas, the fillings and garnishes.