Heat the oil in a large frying pan or wok, and add the chopped vegetables (onion, peppers, courgette, mushrooms). Cook for 5 minutes over a medium heat, stirring regularly, until the vegetables are fairly soft. Add the minced garlic and diced halloumi, and cook for 2 more minutes.
Next add the chopped tomatoes, tomato puree, black olives, oregano, paprika, and chilli flakes. Season to taste (remember the halloumi is quite salty, so don't go overboard with the salt). Mix well, and bring to a simmer. Cook for about 10 minutes, until the sauce has become fairly rich and thick.
Meanwhile, add the garlic, parsley, bread, melted butter and olive oil to a food processor, along with a pinch of salt and pepper. Blitz well to give garlic breadcrumbs.
Transfer the halloumi mixture to a casserole dish, and sprinkle over the garlic breadcrumbs. Bake at 190°C (Gas Mark 5 / 375°F) for around 15-20 minutes, or until the breadcrumbs are golden brown and crispy.
Either serve immediately, or reheat the next day for an even richer casserole - straight from the fridge, it will take about 25 minutes at 180°C (Gas Mark 4 / 350°F).