To begin, prepare the filling. In a bowl, mix together onions, grated carrots, capsicum, chopped green chillies, ginger, and fresh coriander leaves.
Add the hung curd with salt, ginger garlic paste, and chilli sauce. Mix well and keep aside. To prepare the sandwich, butter the bread slices on one side. Spread the filling generously on one of the slices. And top it with the other slice, butter side down.
For the tempering, heat a tawa or a griddle and grease it with oil. Pour in 1 tsp of oil. Add in some mustard seeds along with curry leaves as you would for a tadka. Allow them to crackle.
Place the sandwich over the tadka, press it down, and gently move it a bit so that it does not stick. Cook on medium heat until crisp and golden.
Spread some oil on top and turn the sandwich over. Cook until this side is crisp and golden too.
Remove from heat. The Hung Curd Sandwich is ready. Enjoy with ketchup or a dip of your choice.