Roasted parsnips and sweet potatoes with caper vinaigrette

Treat this recipe as a blueprint for an infinite number of roast vegetable dishes. The point here is to lighten up long-cooked veggies with something crisp and fresh. You can use any of your favourite vegetables – swede, potato, carrot, salsify, beetroot, cauliflower – and many other refreshing combinations at the end: chopped herbs such as basil or mint, grated lemon zest, harissa paste, crushed garlic or a mellow vinegar.