Bring a large pot of salted water to a boil and cook the fettuccine according to package instructions until al dente (usually 8–12 minutes); reserve 1 cup pasta water, drain pasta, and set aside.

Pat shrimp dry and season lightly with salt and pepper.

In a large skillet over medium-high heat, add olive oil and 1 tablespoon butter. When hot, add shrimp in a single layer and cook 1.5–2 minutes per side until pink and just cooked through; remove shrimp and set aside.

Reduce heat to medium and add remaining 1 tablespoon butter. Sauté shallot for 2–3 minutes until softened, then add garlic and cook for 30–45 seconds until fragrant.

Pour in white wine (if using) and simmer for 1–2 minutes until reduced by half. Add stock and simmer for 2–3 minutes.

Stir in heavy cream and bring to a gentle simmer. Cook for 4–6 minutes, stirring occasionally, until sauce begins to thicken. Add cream cheese if using and whisk until smooth.

Stir in grated Parmesan a little at a time until melted and silky. If sauce is too thick, add reserved pasta water, 1/4 cup at a time, until you reach desired consistency. Simmer for 1–2 more minutes.

Add lemon zest, lemon juice, and red pepper flakes (if using). Taste and adjust salt and pepper.

Toss the cooked fettuccine and shrimp into the sauce, heating together for 1–2 minutes so flavors meld. If adding greens or peas, fold them in now to wilt and warm.

Serve immediately with extra Parmesan, chopped parsley or basil, and lemon wedges on the side.