First let us see how to prepare Onion Tomato Chutney. Heat oil in a pan in a medium heat and add urad dal. Let it start getting brown.
When it is almost golden brown, add the red chillies and saute for 5 to 6 seconds.
Add the onions, garlic and fry until the tomatoes are just translucent. Add salt and a pinch of turmeric powder.
Now add the tomatoes and cook till the tomatoes are well cooked and mushy. Turn off the heat and allow to cool.
Add this in to a blender and blend in to a smooth paste. The chutney is ready. Keep aside.
Now, let us make the potato masala. Roughly mash the boiled and peeled potatoes and keep aside.
Heat oil in a pan and crackle mustard seeds. Add urad dal and fry till golden. Now add the curry leaves and red chillies. Saute for 5 seconds.
Add onions, green chillies, ginger, salt and turmeric powder. Cook till onions are translucent. Add tomatoes and cover the lid. Let the tomatoes become mushy.
Add the boiled and mashed potatoes to this and give a good mix until everything is combined. As we are going to add this in to dosa, the masala should be thick. So be careful if you want to add water in this stage. Add very litlle water.
Turn off the heat and sprinkle some coriander leaves.
Now, keep the things ready for dosa. The Dosa batter, Onion Tomato chutney for the base, Potato masala, Idli gun powder, finely chopped onions, coriander leaves, shredded coconut and oil.
Heat a dosa tawa and drizzle some oil on it. Spread the oil all over the tawa using a kitchen tissue or a clean towel. Heat should be high at first.
When it is almost hot, reduce the heat to medium and pour a ladle full of dosa batter on it. Spread the batter in a circular motion in order to get a thin dosa.
Cover a lid and cook for a minute. Keep the heat medium to low. Once the top part is almost cooked, spread the onion tomato chutney all over the dosa.
Take a lemon sized portion from the masala filling and place it in the centre of the dosa.
Now, sprinkle the idli gun powder, chopped onions and coriander leaves. Cover the lid and let it cook for half a minute.
Make sure, in the entire process, the dosa should not get charred at the bottom. So, keep the heat from medium to low from the time you start pouring the dosas.
Carefully separate the dosa from the tawa using a perfect ladle and transfer it to a plate.
Serve immediately with some coconut chutney and Enjoy this Royal Open Masala Dosa.