Melt the butter in a large pan over medium heat. Add the shrimp, red pepper flakes, and garlic. Cook for about 3-4 minutes until the shrimp are cooked.
Remove the shrimp and peel them. Cut the flesh into chunks and add the shells and heads to a fresh pan with cold water.
Strain the butter, reserving the garlic slices.
Return the infused butter to the pan and add the leeks. Sauté until softened but not colored, about 6-8 minutes.
Bring the pan of water with shrimp shells to a boil. Simmer, then strain, discarding the shells and keeping the liquid.
Bring the liquid back to a boil and cook the linguine according to packet instructions. Drain.
Toss the linguine with the leeks, shrimp flesh, reserved garlic, creme fraiche, and chopped parsley.
Serve immediately with lemon wedges on the side to squeeze over.