Chop onion, tomatoes, green chili and curry leaves. Chop onion coarsely, this gives texture to the sandwich stuffing.
Heat cooking oil in the pan, add mustard seeds, fennel seeds, and urad dal. Allow mustard seeds to splutter. Then add chopped onion and curry leaves.
In simmer to medium flame, cook onion by stirring them occasionally.
Once onion slightly turns pink and translucent, add green chilies and chopped tomatoes.
In medium heat, continue to saute them until tomatoes slightly loosen it shape. At this point add turmeric powder and toss them for a second. Add salt and combine everything well.
Heat pan over the stove. Drizzle ½ a teaspoon of butter. Place 2 slices of bread side by side.
Cook both the sides until they turn slight golden brown in color.
Spread tomato+onion masala over the bread and close it with another bread.
Slice them into half if needed. And serve immediately.