Cut off 1 oz (28 g) bottarga, remove its membrane, slice it paper thin, then chop very fine. Rock the knife over it to grind it down to soft grains.
Put scallions or onion in a small saucepan with a large pinch of salt and 1 1/2 tbsp butter (20 g). Cook over medium heat, stirring now and then, until lightly colored.
Drain the cooked pasta and put it in a warm serving bowl with 1 tbsp butter (14 g). Add the contents of the saucepan and toss 2 or 3 times. Add parsley, lemon peel, optional chili, and the bottarga, toss thoroughly, and serve at once.