Cook the spaghetti in a large pan of lightly salted water for 3–5 minutes or according to the packet instructions until just tender. Drain and return to the pan, reserving 3 tablespoons of the cooking water.
Meanwhile, heat the oil in a frying pan with the garlic and warm gently for 2–3 minutes until softened but not coloured. Pour over the cooked spaghetti with the reserved cooking liquid and lemon juice. Season with pepper. Heap into warmed bowls and serve with grated Parmesan.