To make the dough, combine the flour, baking powder, a good pinch of salt and the ajwain seeds (if using) in a large bowl. Use your fingers to rub in 120ml of oil until the mixture resembles breadcrumbs. Stir in 150ml of cold water, then bring it together with your hands into a rough dough, adding a splash more water, if needed. Place in a flour-dusted bowl, cover with a damp tea towel and leave to rest for 30 minutes.
Meanwhile, peel and chop the potatoes into rough 2.5cm chunks, then break the cauliflower into florets so they’re roughly the same size. Add the potatoes to a large pan of boiling salted water and cook for 10 to 12 minutes, or until tender, adding the cauliflower after 3 minutes. Add the peas for the final minute, then drain.
Meanwhile, peel and finely chop the onion and garlic, then peel and finely grate the ginger. Deseed and finely chop the chilli. Heat a lug of oil in a large non-stick frying pan over a medium heat, then add the cumin and onion. Fry for 8 minutes, or until softened but not coloured, stir in the garlic, ginger and chilli, then fry for a further few minutes. Bash the fennel seeds to a fine powder and add to the pan along with the garam masala and ground coriander. Mix well.
Stir the cooked veg into the pan and crush gently with a potato masher – don't overdo it: you want a fairly chunky mixture. Squeeze in the lemon juice and season to taste. Leave to cool, then pick, finely chop and stir in the coriander leaves.
Preheat the oven to 180ºC/350ºF/gas 4. Lightly grease a large baking tray with oil.
On a flour-dusted surface, halve the dough and roll out each portion to about 3cm thick. Using a 7cm pastry cutter, stamp out as many circles as you can, then roll into rounds, roughly ½cm thick and 18cm in diameter. Cut each round in half, then brush the straight edges with a little water. Roll into a cone shape, bringing the straight edges together and pressing lightly to seal. Spoon in the filling, brush the exposed dough with a little water, then fold over and press to seal.
Place onto the prepared baking tray, then repeat with the remaining ingredients, keeping the samosas covered with a damp tea towel as you go. Brush with a little oil and place in the hot oven for around 45 minutes, or until golden and piping hot through. Serve straightaway with mango chutney.