Cook the spaghetti in salted water until just al dente. Squeeze the lemon. Finely dice the onion and garlic, chop the parsley. Drain the sardines in a sieve, roughly shred on a plate, mix with 2 tbsp. lemon juice, 1 1/2 tbsp. parsley and pepper. Toast the pine nuts in a non-stick pan without adding any fat and set aside on a plate.
Heat 2 tbsp. of oil in the pan, toast the breadcrumbs for 1-2 minutes until golden brown, stirring well to prevent them from burning. Remove from the pan, season with salt and pepper. Heat 1 tbsp. olive oil in the pan, fry the onion and garlic over a low heat until translucent, then add the rest of the parsley and the remaining olive oil. Keep some of the pasta water. Drain the spaghetti and immediately add to the pan, mix with the oil and pour in the cooking water. Stir in the sardines and pine nuts and heat briefly. Season with salt and lemon juice and serve sprinkled with breadcrumbs.