Cook pasta in a large pot of boiling water until al dente; drain.
Meanwhile, melt butter in a large saucepan. Saute onion and garlic over medium heat until tender. Stir in half-and-half. Cook, stirring constantly, until sauce thickens slightly (watch carefully; when it begins to look like it will foam, it's done). Add thawed, cooked shrimp just as sauce begins to thicken.
Toss sauce with pasta, Parmesan, pepper and parsley. Let pasta sit for 5 minutes to thicken sauce. Garnish with parsley and lemon slices, if desired.