1 pound cleaned medium shrimp, chopped into 1/2-inch pieces
1 cup thawed frozen peas
2 tablespoons capers, plus 2 tablespoons caper brine
3 tablespoons lemon juice
2 tablespoons chopped parsley, plus more for garnish
Bring a large pot of salted water to a boil over high. Cook spaghetti according to package instructions until al dente. Reserve 1 cup of the pasta cooking water, then drain pasta.
While the pasta is cooking, heat the oil in a 12-inch nonstick skillet over medium and melt 1 tablespoon of the butter in it. Add shallot and cook, stirring, until softened, about 3 minutes. Stir in garlic until fragrant, 30 seconds.
Reduce heat to medium-low, add shrimp and season with salt and pepper. Cook, stirring occasionally, until shrimp just turn opaque, 4 to 5 minutes. If the shrimp is done before the pasta finishes, remove the skillet from heat.
Return pasta and 1 cup of the reserved pasta water to the large pasta pot and heat over medium. Add the shrimp mixture, peas, capers, caper brine and the remaining 3 tablespoons butter, and season with salt and pepper. Stir vigorously until sauce thickens, about 2 minutes. Remove from heat and stir in the lemon juice and parsley.
Divide pasta in bowls and garnish with more parsley.