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 teaspoon 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.
Place the eggplant slices in a bowl, then add the turmeric, lemon juice, cayenne or paprika and 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 the eggplant curry with coriander leaves.