Chop the onion and fry in a little oil. Add the lentils and fry them briefly. Top up with the vegetable stock and simmer until the lentils are cooked through.
While the lentils are simmering, peel the zucchini and cut into spaghetti with a spiral cutter. Alternatively, you can cut the zucchini with a paring knife.
Lightly fry the zucchini in a pan with a little oil and season with salt.
Now add the chopped tomatoes to the lentils and bring to the boil. Add garlic, balsamic vinegar and cayenne pepper and stir well. Finally season with salt and pepper and serve.