Add all the ingredients, except lemon juice and water, for the chutney into the small jar or a chutney jar of a mixer grinder.
Heat oil in a kadai over medium heat. Add mustard seeds and curry leaves, and allow to splutter with ginger garlic paste, green chillies, turmeric powder, red chilli powder and coriander powder. Saute for 1 minutes or until fragrant.
Add salt and mashed potatoes and green peas and mix to combine. Allow to cook for 2 minutes. Add chopped coriander leaves and combine. Set aside.
Using a spoon or butter knife, apply a spoonful of chutney on two slices of bread.
On one slice, assemble the sandwich with a spoon of aloo masala
Add sliced tomato, cucumber, onion and capsicum.
Sprinkle some sandwich masala and cover with the other slices.
Spread some butter on the outside part of both slices. Grill over a pan or use a sandwich maker.
Toast until golden brown on both sides, about 2-3 minutes.
Cut and smear with some more butter, chutney, ketchup and top it off with some sev. Serve hot!