Preheat the oven to 400 degrees Fahrenheit. Dice the cauliflower and broccoli into even-sized florets. Slice the bell pepper into 1 inch strips and slice the onion into 6 segments. Arrange the prepared vegetables across a shallow oven tray. Drizzle over 2 tablespoons of olive oil and season with a good pinch of salt and pepper. Use your hands to coat the vegetables in the oil and seasoning. Transfer to the oven and bake for 10 minutes.
While the vegetables are baking, slice the halloumi into 6 even strips. Remove the part cooked vegetables from the oven. Arrange the halloumi strips in amongst the vegetables. Return to the oven for a further 10-12 minutes or until the vegetables are tender and the cheese is hot and turning golden.
While the cheese is cooking, you can prepare the sauce. Crush the garlic and add to a small bowl. Add the tahini, lakanto, a tablespoon of olive oil, the lemon juice and a good pinch of salt. Whisk together to combine until smooth. Add two tablespoons of water and whisk to thin the dressing. Taste and adjust seasonings as needed and add more water if you would like a thinner dressing. Drizzle the dressing over the warm vegetables and cheese to serve.