Cook the pasta, then drain and return to the pan. Meanwhile, melt the butter in a frying pan over a medium heat and add the onion. Cook until lightly golden, then add the garlic and cook until fragrant. Stir the lentils, onion and garlic, lemon zest and juice, coriander and yogurt through the cooked pasta. Finish with plenty of freshly ground black pepper and serve on warm plates.