Set the oven to broil. Pierce eggplants several times with a fork and place on a rimmed baking sheet. Broil, turning every 5 minutes, until skin is blackened all over and flesh is very soft, about 40 minutes. Remove from oven and let stand until cool enough to handle. Place bell pepper and tomato on a rimmed baking sheet and broil for 20 minutes on each side, or until roasted. Remove tomato and bell pepper from oven, place in a plastic bag and seal the bag. (This helps to remove the skin faster.) When eggplants are cool enough to handle, peel off and discard blackened skin and place flesh in the bowl of a food processor. Peel skin off the roasted tomato and bell peppers, chop them and add them to the food processor bowl. Pulse briefly to combine. In a nonstick skillet over medium heat, add olive oil, minced garlic, harissa and the vegetable mixture. Sauté for few minutes and add salt. Add cayenne, if using. Stir gently for an additional 3 minutes on medium-low heat and place in a serving bowl. Decorate with parsley leaves and serve with toasted flatbread.