Chop onion into small pieces. Crush green chillies in a chopper or mixer. Heat oil in a pan. When oil is hot, add the sliced onions, curry leaves, and salt. Adding salt helps the onion to get cooked fast.
While it gets sauteed, cook potatoes in a separate pan or cooker. If you are using the cooker, switch it off after 2 whistles or until it is done.
Crush or grind ginger and garlic. Once onion turns light brown, add ginger-garlic mixture to it. Once it is sauteed well, add fennel powder, pepper powder and garam masala.
Saute it for a couple of minutes and then add cleaned ground beef. Cook it until beef is done. Ground beef gets cooked soon. It might just take around 10 to 15 minutes. Saute it well and make it dry. Refer notes if you don't want to use ground beef.
Allow the beef mixture to cool down. Meanwhile, peel and mash the cooked potatoes well. Add the mixture to mashed potatoes. Mix it well. Add salt and pepper powder if needed at this moment.
To fry - Make round balls and roll it into oval or other desired shapes. Beat egg well. Dip each cutlet in egg and then roll it in bread crumbs.
Heat oil in a deep fry pan or kadai. Deep fry the cutlets until it's brown. Do it in batches. It would just take 2 to 3 minutes per batch. Use enough oil. Otherwise, you will find the cutlets cracking in the middle. Mistakes that I've done :) You will get around 20 medium sized cutlets with this measurement. Serve it with tomato sauce. Enjoy :)
If you are not using ground beef, pressure cook beef with salt, turmeric powder,pepper powder, garlic and ginger. Keep it for two whistles or until meat is cooked. And then evaporate the water and mince the cooked beef in a mixer. Grind it just enough so that you can feel the meat strands. Do not make it into a paste. Add it to the onion mixture in Step 4.
Mutton cutlet can also made in the same way using ground mutton.