Cook pasta in a large saucepan of boiling water until al dente; drain.
Meanwhile, melt butter in a large saucepan. Saute onion and garlic over medium heat until tender. Add prawns; saute over high heat for 1 minute, stirring constantly. Stir in light cream. Cook, stirring constantly until sauce thickens.
Place pasta in a serving dish and cover with prawns sauce. Sprinkle with black pepper and Parmesan cheese. Garnish with parsley and lemon slices.