1. Thoroughly wash the dal, rice and fenugreek, then drain and place in a blender. Top up with 900ml of fresh water and leave for 6 hours with the lid on to soak (transfer to a covered bowl and work in batches if you have a small blender), then blitz. Leave overnight at room temperature to ferment, then blitz again until super-smooth – this batter will be good for 2 days in the fridge.
2. When you’re ready to cook, preheat the oven to 180°C/350°F/gas 4. Rub the onions, peppers, potatoes and sweet potatoes with 1 tablespoon of oil, then place in a snug-fitting roasting tray. Roast with a splash of water for 1 hour, or until soft, then remove.
3. Peel the ginger, then finely grate just 1cm with the tomatoes and scrape into a second bowl, seasoning with sea salt and black pepper. Peel the garlic, then finely chop with the remaining ginger and the chillies. Gently fry the cumin and mustard seeds in a large non-stick frying pan on a medium heat with the butter for 1 minute. Add the garlic, ginger and chillies, fry for 1 minute, then tear in all the roast vegetables, discarding any tough skins and seeds, and saving any juices from the tray, to serve. Mix and mash everything together, season to perfection with salt and pepper, pick in the coriander leaves, and fry until golden. Keep warm while you make the dosas.
4. Place another large non-stick frying pan over a medium heat, pour in a ladle of batter and swirl around to create a thin layer. Cook for 5 minutes, or until crispy and golden on just one side, then roll up and repeat. Serve with the veg and dips.

Recipe adapted from Veg by Jamie Oliver, which is published by Penguin Random House © Jamie Oliver Enterprises Limited (2019 Veg).