Heat the neutral oil in a casserole and add the onions to sweat gently. Strip and roughly tear the leaves of the chard (keep in the fridge for later), then finely chop the stalks and add to the onion. Cook both until translucent, then add the garlic and ginger and continue cooking until fragrant.
Add all of the ground spices to the pan as well as 3/4 tsp salt and cook for about 30 seconds, just to wake up the flavours.
Tip in the plum tomatoes, breaking them up with a spoon as you go. Refill the tomato tin with the oat milk (one tin is 400 ml), and add to the pan along with one tin of lentils including the liquid. Bring to a boil and cook covered for 1 hour while stirring every now. Take off the lid and keep cooking until the lentils begin to disintegrate and thicken (30 minutes).
In the meantime, make the topping. In a frying pan, fry the mustard seeds until they begin to pop. Add some neutral oil as well as the remaining onion and a pinch of salt and fry on medium-high heat until the onions begin to brown. Add the garlic and sliced red chilli and cook until both are fragrant and the garlic slices are lightly golden. Add a splash of water along with the sugar and cook it into a light syrup. Then turn off the heat.
Trim the lower part of the coriander stalks and discard. Reserve a few leaves, then chop the rest. Add the remaining tin of lentils including the liquid along with the chopped coriander and the chard leaves. Cook for another 5-10 minutes until the chard is wilted. Add the lemon juice and stir through. Then serve with the topping and a few extra coriander leaves.