Light a grill. Brush the eggplant slices and the jalapeño all over with oil and season with salt. Grill the eggplant over moderate heat until nicely charred and tender, about 4 minutes per side. Transfer the eggplant to a work surface and let cool. Grill the jalapeño, turning, until charred and almost tender, about 4 minutes. Peel and seed the jalapeño, then finely chop it. Cut the eggplant into 1/2-inch dice.
In a large bowl, combine the cilantro, vinegar and garlic. Add the eggplant, jalapeño, tomatoes and onion, season with salt and toss. Garnish with the walnuts and serve with grilled flatbread.