Heat the butter in a saucepan. Put the onion and garlic cloves in the fat and sweat until translucent. Add the spring onions with the washed, well-drained red lentils and sauté briefly. Pour in the vegetable stock and simmer over a moderate heat for 10-15 minutes.
Bring everything to a vigorous boil. Then stir in the crème fraîche and add the tomatoes. Cook the spaghetti in salted water with olive oil until al dente, drain, rinse and drain well.
Season the sauce with salt, pepper, sugar, cayenne pepper and balsamic vinegar. Put the spaghetti with the red lentil sauce in a pan, heat, toss and sprinkle with chopped chives.