Cook the pasta in 3-4 l boiling salted water (approx. 1 teaspoon salt per litre) according to package instructions.
Meanwhile, wash the lemon hot, grate it dry and peel the rind in thin strips with a zest ripper. Halve the lemon and squeeze it. Peel and finely dice the onion. Wash lemon balm, shake dry, pluck leaves from the stalks and cut into strips.
Heat butter. Sauté the onion for about 6 minutes. Add the lentils to the onion and fry briefly. Deglaze the lentils with milk and cream, bring to the boil and simmer for about 3 minutes.
Drain the pasta, let it drain and put it back into the pot. Mix lentils with lemon zests, lemon juice and lemon balm. Season to taste with salt and pepper. Add the lentil mixture to the noodles and fold in.
Season again and serve.