In a bowl, combine three-quarters of the bottarga and the lemon juice to create a cream. Gradually stir in the oil to form a thick sauce.
Cook the spaghetti in lightly salted boiling water until al dente. Drain; reserve a little of the cooking water.
Stir the hot cooking water into the bottarga cream to loosen. Add the chilli, and season with salt and pepper. Add the spaghetti to the sauce and toss to coat thoroughly.
Serve immediately, scattered with the remaining bottarga.