For the filling
Heat the oil in a non-stick pan and add the onions, garlic and cook till the onions are translucent.
Add the tomatoes and mushrooms and cook for 2 to 3 minutes.
Add the corn kernels, oregano and salt and cook till the mixture is completely dry. Cool and keep aside.
For the mustard sauce
Mix all the ingredients together and keep aside.
How to proceed
Place one chapati on a dry surface and spread 2 tablespoons of the filling on one side. Roll up the chapati tightly.
Repeat to make 3 more roll ups.
Place the prepared roll ups on a greased baking dish and spoon the mustard sauce over them.
Bake in a pre-heated oven at 220°C (430°F) for 10 to 15 minutes.