To prepare the filling
1. Heat oil in a pan, add the cumin seeds. Once they crackle add the chopped onion. Let onion turn light golden and then add the ginger garlic paste. Cook for 1-2 minutes.
2. Add the pav bhaji masala, salt and red chilli powder. Mix well. Add 1 tbsp of water and cook for 30 seconds.
3. Now add the mashed potatoes, lemon juice and green coriander.
4. Mix everything well and cook for 1-2 minutes. Remove from heat and keep aside.