Soak the lentils for at least 30 minutes, then drain.
Thinly slice the garlic and leek. Cut the dudhi, courgette and red pepper into bite-sized pieces.
Put the leek and garlic into a heavy-based pan , squirt it with oil, then sauté it over low & under cover heat for 10 minutes, until the leek starts to fall apart. Add the courgette , pepper and dudhi and a splash of water, then cook for another 8-10 minutes under cover.
Meanwhile measure out the spice mixture.
When the veggies are tender, add the spices and stir until the mustard seeds start popping. Then add the tinned tomatoes. Rinse the tin with water and add this to the curry.
Add the mung lentils, bring to the boil, cover and gently simmer for 20-30 minutes until the dhal is tender and the sauce has thickened.
Serve warm.