Heat oven to 240C/220C fan/gas 9. Tip the pizza mix into a bowl and make up following pack instructions. Cover the bowl with a cloth and leave for a few mins.
Meanwhile, mix the tomato purée with 4 tbsp water, 1 tbsp oil from the tuna and some seasoning.
Knead the dough for a few mins on a floured surface, then halve and put each piece, spaced apart, on a large oiled baking tray. Press with an oiled hand to make thin flat pizzas - they don't have to be perfectly round.
Spread with the tomato purée mixture, then top with the tuna, capers, cheese and olives. Bake for 10-12 mins. Serve scattered with the onion and rocket.