Pat the shrimp very dry with paper towels (this is what gets you a sear instead of a steam).
Melt the butter in a large skillet over medium-high heat. Add the garlic and red pepper flakes; cook 30 seconds until fragrant.
Add the shrimp in a single layer. Season with salt and pepper. Cook 2 minutes without moving.
Flip and cook 1 to 2 minutes more, until pink and curled into a C shape (an O shape means overcooked — pull them sooner next time).
Squeeze the lemon juice over everything, toss with parsley, and serve immediately over rice or with bread.