Wash the lemon well and grate the zest, avoiding the white part which is very bitter. Remove the remaining zest and cut the lemon into thin slices.
In a non-stick pan, melt the butter over medium-low heat, then add the sliced lemon and cook until golden brown.
Add sugar and cook until caramelized.
Finely slice the onion and fry it in a large pan with the oil until golden. Season with salt.
Pour in the wine, let it evaporate and then add the lemon sauce.
Cook the tagliatelle in boiling salted water. Drain them 2 minutes before the time indicated on the package, reserving a little cooking water and add them directly to the pan with the sauce.
Sprinkle with chopped fresh parsley and, stirring constantly, stir the noodles until cooked, adding the cooking water if necessary.
Serve immediately with a sprinkling of pepper and grated lemon zest.