Slice the stem end off of the eggplant. Slice the eggplant into 1-inch-thick rounds.
To avoid the bitterness salt the eggplant before cooking – lay the eggplant out on a wire rack set over a baking sheet. Sprinkle each side of the slices with the salt and let the eggplant sit for half an hour. Arrange a rack in the middle of the oven and heat to 400°F.
Rinse the eggplant under cool water and then dry thoroughly. Pat dry with paper towels.
Transfer the eggplant to a baking sheet and spread out into a single layer. Combine oil with cumin, brush the both sides of eggplant slices and roast the eggplant for 20 minutes. Flip and roast until golden-brown and tender, 10 to 20 minutes more.
Mix a little bit of water with the Garlic Tzatziki to thin it out.
Warm up the naan bread. Layer the eggplant slices, tomato and onion. Drizzle with tzatziki sauce.