Blanch the tomatoes in boiling water, drain, remove the skin, seeds and any juice and dice the pulp; place the tomato in a sieve and leave to drain for about 10 minutes. Peel the onion, wash, dry and slice finely. Wash the rocket under cold water, drain and pat dry with a cloth and cut into strips. Pour the lemon juice into a bowl, add a pinch of salt and pepper and mix the ingredients until the salt has completely melted; add a drizzle of oil and beat gently with a whisk or with a fork until the sauce has blended completely together. Place the drained diced tomato in a bowl, add the sliced onion, pour over the sauce previously prepared and mix gently with a wooden spoon. In the meantime, bring a saucepan of salted water to the boil and cook the spaghettini until they are “al dente”; drain, run under cold water to cool, transfer to the bowl with the tomato sauce; add the strips of rocket, mix and serve the spaghetti cold.