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. Add shrimp; saute over high heat for 1 minute, stirring constantly. Stir in half-and-half. Cook, stirring constantly, until sauce thickens.

Place pasta in a serving dish, and cover with shrimp sauce. Sprinkle with black pepper and Parmesan cheese. Garnish with parsley and lemon slices.