Peel and chop the onion, finely chop the garlic, and scrub the potatoes if needed. Leave the skin on and chop the potatoes into bite-sized pieces. Drain and rinse the lentils.
Heat 2–3 tbsp of oil in a large, flat pan over medium heat. When hot, add the onion and a good pinch of sea salt, and cook for 5–6 minutes until the onion becomes translucent. Add the cumin seeds, mustard seeds, garlic, pesto, and tomato purée. Mix well and cook for a few more minutes.
Add the potatoes and mix well. Pour in the stock, bring to a boil, then reduce to a simmer. Cover with a lid and cook for 10 minutes. Next, add the courgette, cover with the lid again, and cook for a further 5 minutes (or until the potatoes are cooked through but not too soft). Add the lentils to the pan and stir through to heat for 2–3 more minutes.
Chop the fresh coriander and mix it through the yoghurt. Squeeze in the lime juice and mix well.
Serve with the coriander yoghurt and some toasted wholemeal pitta, naan, or flatbread.