Heat a skillet on medium-high heat. Add oil, mustard seeds and curry leaves. When the mustard seeds begin to splutter, stir in chopped serrano chilies and minced ginger. Saute for a minute.
Add salt, turmeric, cayenne, coriander, cumin and chaat masala and saute for 30 seconds.
Add mashed potatoes and stir to mix well with the spices.
Stir in cooked peas, cilantro and mix to combine. Check for seasoning and turn off the heat.
Assemble Bombay Toastie
Spread butter on bread slice and invert it on the cutting board so that the buttered side is down. Repeat with all slices.
Spread ketchup and green chutney on the other sides. Repeat for all bread slices.
Add 2 to 3 tablespoons of the potato filling and spread it evenly on the slice.
Layer with a few slices of green pepper and onion, followed by tomato. Sprinkle some chaat masala on top.
Layer a slice of cheese. Place the second slice of bread on top, butter side facing up. Assemble the remaining sandwiches.
Grill the sandwich in a panini press or a non-stick skillet for around 4 minutes on each side, or until golden brown on both sides. Cool it for 2 minutes, then slice it in half.
Optional Step: Dress Up the Sandwiches
Slice the sandwich in half, lengthwise or diagonally.
Spread a little chutney or ketchup on the cut side of the sandwich.
Dip it in a bhujiya or your favorite Indian namkeen mixture.
Serrano chile and cayenne are optional. This will create a potato filling around medium spice level. Keep in mind the bread and cheese will level out the spices. That being said, you can always skip the spicy chilies for mild.
Mash and prep the potato filling ahead. Store boiled potatoes, plain mashed potatoes or the complete potato filling in an airtight container in the fridge for up to 4 days.
Make it Vegan. Replace the butter with Vegan butter or spread and use Vegan cheese. Alternatively, omit the cheese completely.
Go for low carb. Wash, trim and boil one head of cauliflower florets until tender, then mash as you would potatoes. Use this in replace of the potatoes for the filling.
Note: The nutrition facts below are my estimates and do not include condiments to serve, bhujiya or namkeen. If you are following any diet plan, I recommend cross-checking with your preferred nutrition calculator.