In a large zip-top bag, combine the olive oil, smashed garlic, lime juice, orange juice, 1/4 cup cilantro, horseradish and crushed red pepper. Add the skirt steak and seal. Marinate for at least 2 hours at room temperature or refrigerate overnight. (If refrigerated, let come to room temperature before grilling.)
Remove steak and use a paper towel to pat the surface somewhat dry. Get your grill extremely hot, and brush with small amount of vegetable oil. Cook for about 3 minutes, flip, and repeat. Surface should be charred, while inside should be medium. Transfer the steak from the grill to a cutting board, cover with foil, and let rest for 10 minutes.
Mash together the avocado, shallot and minced garlic in a mixing bowl. Add salt and pepper along with a squeeze of lime. Set aside.
Cut the steak across the grain into slices, then cut in the opposite direction into bite-size pieces.
To assemble the tacos, warm your tortillas on the grill and fill each tortilla with steak, avocado mixture and queso fresco. Top with sour cream, cilantro and a squeeze of lime, if desired.