Light a grill fire. Grill the eggplant over the open fire or gas flame, turning them frequently, until the skins are black and parched and the flesh feels soft when pierced with a fork, about 10 minutes. When cool enough to handle, peel the charred skin and put the flesh in a colander. Sprinkle with the lemon juice and let stand for 10 minutes.
Meanwhile, grill the bell pepper until it's soft and blackened on all sides, about 15 minutes. Seal the pepper in a paper bag for about 15 minutes. to cool, and peel when cool. Remove the seeds; peel and chop the flesh. Gently squeeze the liquid from the eggplant and transfer the pulp to a mortar or a food processor, along with the bell pepper and chiles. Grind or pulse to a coarse purée. Stir in the garlic, yogurt, walnuts, vinegar, and olive oil; taste and add salt and more vinegar if you like.
Mound in a serving bowl and let stand at room temperature for several hours. Garnish with minced parsley, if you like, and serve at room temperature with crusty bread, pita triangles, or crackers.