Preheat the oven to gas 5, 190°C, fan 170°C. In a large bowl, toss the butternut squash, peppers and red onions with the harissa until completely coated.
Arrange the vegetables in a single layer on 2-3 baking trays or in a large roasting tin and season well. Roast in the oven for 30 minutes, until the vegetables are golden and crisp at the edges.
Meanwhile, combine the Greek yogurt with the mint and a little seasoning. To serve, dollop a few spoonfuls of yogurt over the roasted vegetables. Serve with couscous and the remaining minty yogurt on the side.