Add onions and sauté till onion become translucent and soft.
Add fresh chopped or grated ginger, garlic and sauté for few seconds. You can add ginger garlic paste also.
Add capsicum and sauté for a minute on medium flame.
Add Tomatoes, salt and mix it well. Cook tomatoes till it becomes soft and mushy.
Once tomatoes become soft add all the spices (Turmeric powder, red chilly powder, pav bhaji masala) and mix it well.
Add boiled and mashed potatoes and mix it well till all the spices gets blended well.
Mash all the veggies with spatula or potato masher.
Lastly add coriander leaves, spring onions green part, and lemon juice, mix it well.
Bhaji is ready.
Take one piece of left over roti and spread bhaji evenly all around.
Grate some cheese over the layer of bhaji and arrange tomato roundels.
Cover the top with another piece of left over roti to make sandwich.
Put butter in a pan and let the butter melts.now place pav bhaji sandwich, cover the lid and let it cook from downwards for 2 minutes on medium low flame
After 2 minutes uncover, spread butter on top and flip it over.
Again cover the led and let pav bhaji sandwich cook from other side too.
Uncover the lid and take out Pav Bhaji Roti Sandwich onto plate.