Heat oil, add green chilies, cabbage, bell pepper and saute for a minute or two.
Sprinkle 3 - 4 tablespoon of water, cover and cook on medium flame until cabbage is very soft.
Uncover and saute for few minutes to let moisture evaporate.
Let cabbage mixture cool.
Mix mayo, mustard powder, black pepper and cilantro.
Spread the stuffing on one slice of bread, cover with the second. Repeat the same with other two slices.
Trim the edges and cut in half. This is optional.
Refrigerate until ready to serve.
For hot sandwiches, grill sandwiches on a pan or in a sandwich maker and no need to trim the edges.
Video
Notes
You can steam cabbage and bell peppers instead of sauteing.
Cook or steam cabbage and bell peppers a day before and store in the fridge. Mix rest of the ingredients just before making sandwiches.
Use spices and herbs of your choice. Feel free to replace green chilies and mustard powder with black pepper, hot sauce and dijon mustard.
For hot sandwiches, grill sandwiches in a pan or sandwich maker.
Cream cheese is a good substitute for mayonnaise. I used Philadelphia cream cheese once. Feel free to use cream cheese of your choice. Thick yogurt or Greek yogurt is also a good substitute for mayo.