To roast garlic: Preheat oven to 325 degrees. Cut top quarter from each head of garlic. Place garlic, cut side up on sheet of aluminum foil. Drizzle with olive oil and sprinkle with salt and pepper. Close foil tightly over garlic leving in a single row. Roast garlic until golden brown and very soft (about 1 hour). Once cooled squeeze into recipe below.
Melt butter over medium heat in large pot.
Add pancetta and cook until fat is rendered and add, leeks, onions, salt, white pepper and thyme and cook until vegetables are soft but not browning (about 5 minutes).
Add roasted garlic and mix well.
Add wine, bring to a boil, and simmer while stirring until wine is almost evaporated (about 3-5 minutes).
Add the chicken stock and bring to a boil. Reduce heat and simmer, stirring occasionally, until the vegetables are very soft and the soup is slightly thickened (about 25-30 minutes),
Add cubes of bread to thicken, if desired, then blend with immersion blender or in food processor until smooth.
Add cream, parsley and lemon juice and simmer for 5 minutes. Adjust seasoning to taste and serve with good croutons or crusty french bread.