Heat olive oil over medium-low heat in a large saucepan. When hot add the onions and garlic and cook until the onion is translucent, about 4 minutes. Add the parsley, chives, tarragon and lettuce and stir until the lettuce is completely wilted, about 3 minutes. Add the chicken stock and simmer, uncovered, for 20 minutes. When ready to serve, process the soup, in batches, taking care since the soup is hot, then return to a clean saucepan. Stir in the heavy cream or evaporated milk and the salt and pepper and simmer for another 5 minutes, or until heated through. Adjust seasoning, if necessary, and serve immediately, garnished with fresh herbs and chive blossoms, if desired.