Cook the spaghetti in a large saucepan of salted boiling water for 10 minutes, adding the broccoli for the final 3 minutes, or until just tender.
Meanwhile, heat the oil in a frying pan, add the onion and chilli and cook over a gentle heat for 2 minutes. Stir in the capers, lemon rind and juice and vinegar, season with salt and pepper and heat through.
Drain the spaghetti and broccoli, reserving 1 tablespoon of the cooking water and adding to the caper mixture. Add the drained spaghetti and broccoli to the pan and toss well to combine over the heat. Serve with an extra grinding of pepper.