It is easiest to char the eggplants on the stovetop griddle, but you can also roast the eggplants in a high-heated oven until they are fork-tender. Charring them gives the soup a deeper smoky flavor.
Preheat a cast-iron griddle or skillet over medium-high heat. Place the eggplants on the griddle and char them well, turning from time to time, until the outer skin is burnt and peeling and the inside is very soft, about 15 minutes.
Transfer to a bowl and let cool. Trim away the skin and discard. On a parchment-lined cutting board, chop the eggplant and set aside, along with any juice that has accumulated on the parchment.
Meanwhile, combine the onions, garlic, and olive oil in a 3-quart saucepan over medium heat for 5 to 10 minutes, or until they are soft and translucent. Add the chicken stock, herbes de Provence, and bay leaf and bring to a boil. Reduce the heat and simmer for 15 minutes.
Season with salt, pepper, and lemon juice. Remove the bay leaf, add the eggplant, and puree the soup in a blender or food processor. Stir in the buttermilk.
Ladle into soup bowls and swirl in the Red Pepper Coulis. Garnish with the parsley and serve.