Preheat the oven to 180C fan-forced (200C conventional).
Add a generous lug of olive oil to a large flat roasting tray and arrange the eggplant and zucchini slices on top. Some overlap is fine. Scatter with oregano leaves and roast for 20 minutes. The eggplant will take on quite a bit of colour – don't worry about this.
Open the oven and fling the corn and tomatoes over the eggplant and zucchini. Place the slices of halloumi on top of the veg and cook for another 15 minutes.
Change the oven setting to the top grill and cook for another 3-8 minutes to brown the top of the halloumi, keeping a close eye on things.
Remove the tray from the oven, season generously with salt, pepper and lemon zest. Squeeze over the lemon juice and give everything another lug of olive oil. Sprinkle Aleppo pepper or chilli flakes over the halloumi to taste. Scatter over the fresh herbs and a few stems of watercress, if using, and serve immediately.
