Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the sweet potatoes and trim the ends off courgette. Chop both into bite-size chunks. Peel the red onion and chop it into 8 wedges.
Arrange the sweet potatoes, courgette and onion wedges in a large roasting tin. Sprinkle over 1 tsp cumin seeds and a good pinch of salt and pepper. Drizzle over 1 tbsp oil and toss well to coat. Slide into the oven and roast for 25 mins.
Slide the piadinas directly onto the shelf below the roasting vegetables. Cook for 5-10 mins, till golden and crisp. Set aside and allow to cool slightly, then break into bite-size pieces.
While the veg finish roasting, spoon the yogurt into a bowl. Peel the garlic and finely chop or grate it into the yogurt. Sprinkle in a little salt and pepper and stir well to combine. Finely chop the parsley leaves.
Assemble the salad by layering the roast veg, crispy piadinas and garlic yogurt on 2 warm plates, alternating the layers till you’ve used all the ingredients up. Finish the salads with a scattering of chopped parsley and serve.