Cut the pumpkin in half then into slices about 2cm thick, removing the seeds and fibres, but leaving the skin in place. Put the pumpkin slices into the mixing bowl, then toss with the aromatics, so the pumpkin pieces are evenly coated.
Transfer the pumpkin to a foil-lined baking tray, tip over any remaining seasoning and oil then bake for about 40 minutes until soft and patchily brown.
Make the sauce: peel and roughly chop the onions. Warm the oil in a large deep pan, add the onions and let them cook until soft and pale gold. Peel and finely slice the garlic and add to the softening onions.
Roughly chop the tomatoes and stir into the onion and garlic, season with salt then leave to cook over a moderate heat for about 35 minutes, giving the occasional stir to stop the mixture sticking. You might find it helpful to partially cover the pan with a lid.
Put the yogurt in a small bowl. Shred the basil leaves and the mint and stir into the yogurt. When the sauce is thick, serve with the roast pumpkin and spoon over the herbed yogurt.