This fresh-tasting dish can also be chilled and served for lunch. The recipe can be made with many different pastas, including penne or shells. Any leafy green, such as spinach or arugula, can be substituted for the watercress.
In a large pot of boiling salted water, cook pasta until al dente according to package instructions, about 12 minutes. Drain pasta, and return to pot; toss with 1 tablespoon oil. Cover to keep warm.
While pasta is cooking, heat remaining 5 tablespoons oil over medium-high heat in a medium saute pan. Add garlic and white parts of scallions, and cook until fragrant, about 30 seconds.
Add shrimp to pan, and cook 1 minute. Turn shrimp, and cook until pink and opaque, about 1 minute more. Stir in peas, wine, lemon zest, and red pepper; cook 1 minute.
Transfer shrimp mixture to pot; toss to combine with pasta. Drizzle lemon juice over pasta; sprinkle in green parts of scallions and the watercress. Season with salt and pepper; toss gently before serving.
When slicing scallions, remember to keep the green and white parts separate: The white pieces get sauteed with the garlic; the green are sprinkled in just before serving.