Heat half of the oil in a frying pan over medium-high heat. Add the beef and cook until browned on all sides and fully cooked through. Remove the beef from the pan and set aside.
Add the remaining oil to the same pan. Stir in the mustard seeds and allow them to pop. Add the dried red chili and curry leaves and cook for about 30 seconds until fragrant.
Add the sliced onions and cook for 5 to 7 minutes, stirring occasionally, until they begin to soften. Stir in the diced tomato and continue cooking until the tomato breaks down.
Add the turmeric powder, chili flakes, Kashmiri chili powder, and salt. Mix well and cook for another minute so the spices become fragrant. Add the sliced green chilies and continue cooking until the onions become lightly caramelized.
Combine the cooked beef to the pan. Stir everything together and reduce the heat to low. Cover and cook for about 5 minutes so the flavors can blend.
Remove the lid, taste, and adjust salt if needed. Continue cooking for another 5 minutes, stirring occasionally.
