Slice four hard-boiled eggs using a cotton thread or a knife. Put them on a plate aside.
Next, take 2 pieces roughly diced medium-sized onions, 8 peeled garlic cloves, and 2 pieces of large green chilies (broken into small pieces) in a chopper and chop them finely. When done, grate ½-inch size ginger and add to it. Keep it aside. (Pro tip: Typically, choppers cannot chop small pieces of ginger finely. So, it is better to grate it later).
Now, fry the chopped mixture until golden in 2 tbsp mustard oil in a pan on a medium flame. (Pro Tip: Do not use paste of onion/ginger/garlic; always chop them and use. It adds to the taste and crunchiness in every bite).
The, add 1 tsp of cumin powder, 1 tsp of coriander powder, ½ tsp of turmeric powder, 1 tsp of red chili powder and cook them for a minute. (Pro tip: Since no water is added, keep the flame low and stir continuously to prevent the masala from burning).
Next, add 3 pieces of large potatoes (boiled, peeled, and grated), ½ tsp of black salt, ½ tsp of regular salt, ½ tsp of garam masala powder, and 1.5 tsp of spice mix (roasted and ground cumin seeds, coriander seeds, and dry red chilies). Mix and cook for 4 minutes until dry. Now, add a handful of finely chopped coriander leaves and mix it. Transfer it to a plate and spread it with the spatula to cool faster. (Pro tip: The stuffing must be dry, otherwise it will leak out during frying. If it is not as dry as desired, you may add a bit of bread crumbs or crushed biscuits.).
Now, press flat a small portion of the potato mixture on your palm, place one half egg (cut side down), and cover it completely with the stuffing. (Pro tip: If the egg pieces are not covered properly, it will break and fall apart while frying).
Now, in 3 separate containers take corn flour,2 eggs, and bread crumbs. Season them with salt and black pepper powder. Optionally, add a bit of roasted masala mix to the bread crumbs for extra flavor.
Now, coat a piece with corn flour, then with egg mixture, and finally with bread crumbs.
Repeat the steps to double coat. Keep them aside on a plate for about 10 minutes to set properly. (Pro tip: Double coating prevents the chances of breaking while frying the egg devils).
Now, fry the coated egg devils in hot oil. Flip occasionally until golden brown. Remove when done.
Your homemade egg devils are ready. Serve them with mustard sauce and onion rings.