Preheat the grill to 400°F (204°C). Alternatively, you can heat a grill pan over medium-high or preheat the broiler to high. In a medium bowl, whisk together the harissa and olive oil.
Cut the halloumi into 12 to 16 chunks. Add the halloumi to the bowl and toss until coated.
Cut the bell pepper into chunks about the same size as the mushrooms. Thickly slice the onion or cut it into similarly sized chunks.
Thread the halloumi and vegetables onto the prepared skewers, alternating between different ingredients or, to ensure even cooking and the ability to take things off the grill as they’re done, thread the halloumi and individual vegetables on separate skewers. If you’re doing this, make sure to leave a little space between the pieces of halloumi so the edges cook.
Place the skewers on a plate, drizzle with a little olive oil, sprinkle with a pinch of salt, and cook on the preheated grill or grill pan or under the broiler until the halloumi has started to char, then turn once to cook the other side, removing from the heat as soon as the halloumi is golden, 2 to 4 minutes per side. Ideally, you want the halloumi to be slightly charred on the outside yet stringy on the inside.
Stack the vegetable skewers on a platter and, if desired, scatter with the chopped mint and/or oregano. Serve with any accompaniments of your choice, if using.