Preheat oven to 225C/450F. Heat 1 tablespoon olive or rapeseed oil in a large frying pan. Add the onion to the pan along with a pinch of salt and saute for 5-10 minutes until softened.
Add the peppers to the pan and saute for 5-10 minutes until softened. Add the parsley and garlic and cook for another minute.
Add the tomato puree, harissa, balsamic vinegar and 2 tablespoons water and cook for 1-2 minutes until a thick paste has formed. Season with salt to taste.
Spread the mixture over the flatbreads and top with the grated halloumi.
Bake for 5-10 minutes until the cheese is golden and serve immediately.