Start by preheating your oven to 200 C / Gas 6. Find your baking dish, which should be big enough to get all the veggies in without having them stuffed in and almost falling over. There should ideally be at least 1cm left at the top.
Place the sliced potatoes, courgettes and red onions in a big bowl.
Place 10 of the plum tomatoes in a food processor with 3 tablespoons oil, 3 cloves garlic, garlic powder, oregano, thyme and a very generous amount of pepper and salt. Puree till smooth, then add 100ml of water and pour the puree over the bowl of veggies, coating evenly. Transfer to the baking dish.
Place the dish in the middle of the oven and put the timer to 1 1/2 hours. After the first 30 minutes, check every 20 to 30 minutes to ensure the dish is not drying out. You should stir and taste at about 45 minutes to 1 hour, adding more salt and pepper if needed. Sometimes I stir more often than so to ensure it does not burn.
Meanwhile, whisk together 2 tablespoons oil, lemon juice and 1 crushed garlic clove in a bowl. Add pepper, the olives and remaining tomatoes, finely chopped, and set aside to marinate.
Heat a frying pan on medium-high heat. Take some of the marinade liquid (that your olives and tomatoes are in) and fry the halloumi in the marinade. Once it's gotten colour on both sides (2 to 3 minutes per side), then add the olive and tomatoes to the pan and fry for a short time.
Serve the warm halloumi and tomato mixture over a bed of fresh spinach leaves, with the briam on the side.