Start by preparing the avocado horseradish sauce. Place avocado, garlic, lemon juice, horseradish and salt together in a food processor. Process until smooth. Transfer to a bowl, cover and refrigerate while you prepare the tacos.

Heat olive oil in a skillet over medium heat. Cook onions until softened, about 3 minutes. Stir in garlic and sliced prime rib. Cook just until the prime rib is warmed through. Transfer to a plate, cover with aluminum foil to keep warm.

In the same skillet you cooked the meat, heat a thin layer of oil. Fry a taco shell, 30 seconds on each side and transfer to a paper towel lined plate. Repeat with remaining taco shells.

To assemble a taco, place a spoonful of prime rib mixture in the middle of a shell. Spoon avocado sauce on top. Sprinkle with cilantro. Repeat with remaining ingredients. Serve.