To make the dressing, whisk together the olive oil, balsamic vinegar, caster sugar and garlic. Season with a little salt and pepper. Spread the vegetables and tomatoes out in a large roasting tin and pour in roughly half the dressing, tossing well to mix. Roast in the oven for 35-40 minutes, stirring halfway through.
Whilst the vegetables are roasting, cook the quinoa. Rinse well under running cold water then tip into a saucepan. Pour in the stock, set over a low heat and cover, bring up to a gentle simmer and cook for 15 minutes until the grains are tender but still with plenty of bite and the stock has been absorbed. Lightly stir through the rest of the dressing.
Heat a griddle pan until hot and lightly brush the halloumi with a little oil. Grill on both sides for 2-3 minutes until crisp. Remove to a board and roughly chop before adding to the salad.
Once the vegetables are cooked, toss them lightly through the salad and serve scattered with the basil.