Cook the pasta according to the packet instructions, then drain and return to the pan. Keep warm under a lid.
Meanwhile, chop finely the onions and slice thinly garlic. Deseed the chillies and chop the flesh finely.
Heat 1 tablespoon oil in a frying pan and add the onion, garlic and chilli. Stir-fry for 2 minutes until softened.
Drain the tuna and add to the pan with one more spoonful of oil.
Add the parsley and the lemon juice. Stir and cook for about a minute then fold in the spaghetti.
Warm through, season with salt and pepper.
Serve warm, perhaps with some grated Parmesan scattered on top.