In a bowl, mix the gram flour, turmeric, chilli powder, ajwain or cumin seeds, salt and black pepper.
Gradually add the water, whisking continuously, until you have a smooth, pourable batter without any lumps. The batter should be thick enough to coat the back of a spoon but thin enough to spread easily. If it seems too thick, add a little more water.
Add the chopped onion, tomato, red pepper, ginger and coriander leaves. Mix well until everything is evenly combined.
Heat a non-stick frying or pancake pan over a medium heat. Once hot, add a little oil and spread it over the surface with a kitchen paper towel.
Pour in a ladleful of batter and use the back of the ladle to spread it into a small, thin round pancake.
Cook for about 1–2 minutes, until the top has set and the underside is lightly browned. Carefully flip with a spatula and cook the other side until cooked through and lightly browned.
Transfer to a plate and repeat with the remaining batter. Wipe the pan with a little kitchen paper between chillas if necessary, then add a little more oil before cooking the next one.
Serve the besan chilla hot with mint chutney, tomato chutney or another chutney of your choice.