Wrap eggplant in a double layer of aluminum foil, sealing the package well. Set package directly over a medium gas flame or a charcoal fire, turning occasionally until eggplant is tender, about 25 minutes. You can also bake the eggplant unwrapped on a baking sheet in a moderately hot oven, about 400 degrees, pricking it first to prevent bursting, but it will not have the desirable smoky taste.
Carefully unwrap the foil package, and when eggplant is cool enough to handle, remove the charred black skin. Scrape flesh into a bowl and add cold water to cover. Add lemon juice and let stand 30 minutes to keep the eggplant from darkening. Drain eggplant and squeeze dry.
Over a gas flame or charcoal fire, or under a broiler, cook green pepper and chile until blackened and blistered all over. Set aside until cool enough to handle. Remove the skins, ribs and seeds. Mince the green pepper and chile.
Put the eggplant, green pepper, chile and garlic in a mortar and pound to a paste. Alternatively, mince to a paste by hand. Stir in the olive oil, walnuts and yogurt. Season to taste with salt, pepper and wine vinegar. Let stand at room temperature for a couple of hours to blend flavors, or refrigerate for longer keeping.