Preheat the oven to 425°F. Set out a large baking sheet.
Place 4 large pieces of heavy-duty aluminum foil* on the counter (about 18 inches long each).
Evenly divide asparagus between each of the foil packs.
In a large bowl, toss together the shrimp, garlic, salt, pepper, and red pepper flakes (if using) until everything is evenly coated.
Divide the mixture between the four pieces of foil, piling it in the center of each piece.
Drizzle each foil packet with 1½ teaspoons of chicken stock and 1½ teaspoons of lemon juice. Top each with a tablespoon of butter cubes.
Fold the long sides of the foil together over the top of the mixture, turning it over twice to seal the contents. Fold up the ends and seal the packet closed. Leave some room for air to circulate. Repeat for all the packets.
Place the foil packets onto the baking sheet, seam-side up. Bake for 15-20 minutes, or until the shrimp is cooked through. (Shrimp are cooked through when they are opaque pink and curl into a 'c'.)
Open the foil packets carefully and serve hot, garnished with lemon and parsley.