In a large pot, melt the butter over medium-high heat. Add the pancetta and cook until the fat is rendered, about 4 minutes. Add the leeks, onions, salt, pepper, and thyme, and cook, stirring, until the vegetables are very soft but not colored, 5 to 6 minutes. Add the roasted garlic and stir well to incorporate. Add the wine, bring to a boil, and cook, stirring, until the wine is nearly all evaporated, about 3 minutes. Add the stock and bring to a boil. Reduce the heat and simmer, stirring occasionally, until the vegetables are very soft and the soup is slightly thickened, 20 to 30 minutes. Add cubes of bread to thicken, if desired, and then blend with an immersion blender. Add the cream, parsley, and lemon juice and simmer for 5 minutes. Adjust the seasoning to taste.
Meanwhile, preheat the oven to 400 degrees F.
Place the sliced bread on a baking sheet and bake on both sides until just crisp and light golden brown, about 5 minutes per side. Remove from the oven. Rub one side of each with the cut garlic clove. In a small bowl, mash the cheese with the oil and spread 1 tablespoon of the mixture on top of each crouton. Return to the oven and bake until the cheese is melted, about 5 minutes.
To serve, ladle the soup into bowls and top each serving with 2 cheese croutons, Garnish with chopped chives and serve hot.
Preheat the oven to 325 degrees F. Line a baking sheet with parchment paper or aluminum foil.
Cut the top quarter from each head of garlic and place, cut side up, on the prepared baking sheet. Drizzle the oil lightly over the cut garlic tops, and sprinkle with the salt and pepper. Turn the garlic cut side down, and roast until the cloves are soft and golden brown, about 1 hour.
Remove from the oven and let sit until cool enough to handle. Squeeze each head of garlic, gently pressing with your fingers to expel the cloves into a bowl. Stir the garlic with a rubber spatula to blend thoroughly. Use as needed, or store refrigerated in an airtight container.