Bring a large pot of water to a boil and salt it generously. While it heats, pat the shrimp dry and season them with 1/2 teaspoon salt, then finely grate the zest from the lemon and cut the lemon in half.
Add the spaghetti to the boiling water, stir for the first minute so the strands don't stick, and cook until al dente, about a minute less than the package says.
Make the sauce
Meanwhile, put the oil, garlic and chili flakes in a large, cold skillet and set it over medium-low heat. Cook, stirring often, for 4 to 6 minutes, until the garlic is pale gold. Starting in cold oil and heating slowly keeps the garlic from burning and turning bitter.
Raise the heat to medium-high and add the shrimp in a single layer. Cook for 1 to 2 minutes, until pink underneath, then turn them, add the tomatoes and cook for 1 to 2 minutes more, until the shrimp are just opaque and the tomatoes start to soften.
Bring it together
Scoop out 1 cup of the pasta cooking water, then drain the spaghetti and add it to the skillet with 1/2 cup of the reserved water, the lemon zest and the juice of half the lemon.
Toss over medium heat for about 1 minute, until the oil and water come together into a light sauce that clings to the pasta and the shrimp are opaque throughout. Add more pasta water a splash at a time if it looks dry.
Toss in the parsley, taste, and add more salt or a squeeze from the remaining lemon half if needed. Serve right away.
Cook's notes
Frozen shrimp are fine. Thaw them overnight in the fridge, or in a colander under cold running water for about 10 minutes, then pat them dry.
If the garlic starts to brown before the shrimp go in, pull the pan off the heat for a moment; browned garlic turns bitter.
The starch in the pasta water is what turns the oil into a sauce, so don't skip saving it.
If the sauce is ready before the pasta, take the skillet off the heat; the shrimp finish cooking when everything is tossed together.
Best eaten straight away; the shrimp toughen when reheated.
RecipeFix originals are written as practical home recipes and editorially reviewed for internal consistency. They are not described as kitchen-tested. Nutrition is not shown unless it is calculated from the final recipe.