Set aside the frozen mixed peppers for 1 hour to defrost, then pat dry with kitchen paper. Preheat the oven to 200°C/gas mark 6.
Cut the pittas in half horizontally and put on a large baking tray, cut-side down. Spread the tomato purée over the pitta halves, then top with the mushrooms, cheddar, sweetcorn, tomatoes and defrosted peppers.
Cook in the oven for 10-12 minutes, until the cheese is melted and bubbling. Serve two pitta halves per person with the shredded iceberg lettuce.