Boil the pasta. Bring a large pot of heavily salted water to a boil. Cook the fettuccine until just al dente. Reserve about ½ cup of the starchy pasta water before draining.
Sauté the mushrooms. While the water boils, melt the butter in a large skillet over medium-high heat. Add mushrooms in a single layer and cook undisturbed for until deeply golden, then stir and finish browning another minute.
Add aromatics and prosciutto. Lower the heat to medium. Stir in shallot and prosciutto; sauté until shallot is translucent and prosciutto is fragrant. Add garlic in the final .
Deglaze (optional). Add a splash of dry white wine and scrape up any browned bits from the bottom of the pan. Let it cook off, ~.
Build the cream sauce. Pour in the heavy cream and add the peas. Bring to a gentle simmer and cook until slightly thickened.
Emulsify. Off heat, stir in Parmesan until melted and glossy. If the sauce is too thick, loosen with reserved pasta water 1–2 tablespoons at a time until silky.
Combine. Toss the drained fettuccine directly into the skillet, using tongs to coat every strand. Season generously with cracked black pepper. Taste before salting — prosciutto and Parmesan are already salty.
Serve. Plate immediately. Top with extra Parmesan and more black pepper.