To a pan on medium heat, add oil, mustard seeds, and cumin seeds.
Once the mustard seeds sizzle or pop, add chopped onion and saute for 1-2 minutes until onion turns translucent. Then, add chopped tomatoes and saute for another minute.
Next, add salt, red chilli powder, and garam masala and combine everything well.
Now, add milk and saute for 5 minutes on medium high heat until mixture thickens.
Finally, add chopped coriander, mix well and set aside.
Heat a griddle or iron tawa and spread some oil. Place the sliced bun and top it with a thick layer of masala mixture and then place another slice of the bun on the masala mixture.
Sprinkle some oil on top of the bun and using a spatula, flip and toast the stuffed masala bun. Serve hot and Enjoy!