Powder the mustard, coriander, cumin and fennel powders in a spice grinder or blender. Set aside.
Slather the eggplant slices with salt, pepper and ½ tsp oil, and place on a baking sheet. Place the baking sheet under the broiler and cook for four minutes until the slices are reddish-brown on top. Flip over and cook the other side for another 2-3 minutes.
You can also do this in the air fryer: place the eggplant slices in the air fryer basket and cook for 8-10 minutes at 375 degrees or until browned outside and cooked. Toss the eggplant slices once during cooking to make sure they brown evenly.
Cut each eggplant slice into half. Then add the turmeric, lemon juice, cayenne or paprika and the powdered spices. Mix well.
Heat the remaining oil in a saucepan.
Add the onion and fry until lightly brown. Add the curry leaves and stir for a couple of minutes.
Add the eggplant and half the coconut milk. Bring to a boil, lower the heat to a simmer, cover and let it cook five minutes.
Add the remaining coconut milk and warm through without letting the curry come to a boil. Add more salt as needed.
Turn off the heat. Garnish with coriander leaves.