Cook the tagliatelle in salted water according to the instructions on the packet.
In the meantime, peel the onion and cut into small cubes. Sweat in oil in a high pan. Add the spinach leaves and fry briefly. Halve the cocktail tomatoes and add to the pan with the peeled tomatoes. Season to taste with salt, pepper and lemon juice.
Stir in the finished pasta and the crème fraîche. Finally, sprinkle the basil leaves on top to taste.
Dose the lemon juice to taste. Depending on how sour it should be, use a little more or less than stated.