Preheat oven to 500 degrees F.
Spread rock salt in the bottom of a large cast iron skillet, enough to cover the bottom about an inch thick
Place skillet in the hot oven for 5-7 minutes to get really hot, then arrange shrimp (shell-on) on top of the salt
Roast the shrimp on the salt for 6-7 minutes, until just opaque in the center.
Melt butter in a small saucepan over low heat, then add the lemon, garlic and parsley
Stir until garlic is soft, then serve with the roasted shrimp for dipping.