Bring a small pot of well-salted water to a boil and cook the spaghetti until just al dente, about 8-9 minutes. Scoop out 1/4 cup of the pasta water before draining (this starchy water is what makes the sauce cling).
While the pasta cooks, melt the butter in a skillet over medium heat. Add the garlic and red pepper flakes and cook 60-90 seconds until golden at the edges – not brown, or it turns bitter (learned that one the hard way).
Add the shrimp in a single layer, season with salt and pepper, and cook 2-3 minutes, flipping once, until pink and just opaque.
Toss in the drained pasta with the lemon juice and a splash of pasta water. Toss for 30 seconds until glossy, then shower with parsley.