Put 1 1/2 cups besan into a bowl. Add 1 tsp red chilli powder, 1 1/4 tsp salt, 1/2 tsp turmeric powder and 1/2 tsp baking powder and mix. Slowly add 1 cup water whilst whisking, to make a smooth thick paste.
Mix in the onions and spinach. Squeeze out water with hands beforehand if using defrosted spinach blocks. Add potatoes if desired.
Heat a wok full of oil, full heat, 5 mins before frying. Once the oil is hot, use a tbsp to scoop up the mixture and drop one tbsp at a time into the oil, until there is no space left on the surface. Fry on full heat for 5-6 mins, turning half way through.
Once golden on both sides, remove and let drain in a metal sieve or directly on to a plate lined with paper towel. Fry more batches until all the mixture is used up. Alternatively, you can refrigerate the mixture (covered) and use up within a week.