How to Make It
Cook the pasta according to the package directions. Reserve 3/4 cup of the cooking water; drain the pasta. Wipe out the pot.
Heat the oil in the pasta pot over medium-high heat. Add the shrimp, garlic, red pepper, and ½ teaspoon salt. Cook, tossing occasionally, until cooked through, 3 to 4 minutes.
Reduce heat to medium and add the pasta, watercress, lemon zest, and 1/2 cup of the reserved cooking water and cook, tossing, until the sauce coats the pasta, 1 to 2 minutes (adding more cooking water as needed to loosen the sauce).