Step 1: Cook the pasta according to package directions in a large pot of salted water. Before draining, reserve about 1 cup of pasta water. Drain the pasta and set aside.
Step 2: While the pasta cooks, heat the olive oil and butter in a large skillet over medium heat. Once the butter is melted and shimmering, add the minced garlic and red pepper flakes (if using). Sauté for 1-2 minutes until fragrant, being careful not to burn the garlic.
Step 3: Add the thawed and patted dry precooked shrimp to the skillet. Cook for only 2-3 minutes, stirring occasionally, just until the shrimp are warmed through. Remember, they are already cooked, so overcooking will make them rubbery.
Step 4: Pour in the chicken or vegetable broth (or 1/2 cup of reserved pasta water). Bring to a gentle simmer and cook for 1 minute to slightly reduce. Stir in the fresh lemon juice and chopped parsley.
Step 5: Add the drained pasta to the skillet with the shrimp and sauce. Toss everything together until the pasta is well coated. If the sauce seems too thick, add a splash more reserved pasta water until it reaches your desired consistency.
Step 6: Season generously with salt and black pepper to taste. Serve immediately, perhaps with an extra sprinkle of fresh parsley or Parmesan cheese.