Preheat the oven to 200ºC/180ºC Fan/Gas Mark 6.
Place a large casserole on a medium heat and sauté the butter, onion, turmeric and cumin for 10 minutes.
Add the tomato purée and cook, stirring, for another 2 minutes.
Bundle the herbs together and use some string to tie them into a bunch.
Add these to the dish along with the potatoes, squash, limes, chilli, 1½ teaspoons of salt and 1 litre of water and bring to the boil.
Reduce the heat and boil gently for 15 minutes, until the potatoes are semi-cooked.
Stir through the tomatoes, spinach and barberries, crushing the limes gently to release some of the juice inside, and transfer everything to a large roasting tray.
Bake uncovered for 20 minutes, or until the sauce has thickened a little and the vegetables are soft.
Remove from the oven and allow to sit for 5 minutes before serving with a dollop of yoghurt on the side, if you choose.
