Melt 1 tablespoon of the butter in a large skillet. Add the diced baguette and cook over moderately low heat until browned on the bottom, about 3 minutes. Stir the croutons and cook until crisp all over, about 2 minutes longer. Transfer to a plate to cool.
Melt the remaining 1 tablespoon of butter in a large saucepan. Add the bacon and cook over moderately low heat until lightly browned, about 4 minutes. Add the onion and cook until softened, about 7 minutes. Add the wine and simmer until almost evaporated, about 4 minutes. Add the stock and garlic and bring to a simmer. Add the potatoes, season with salt and pepper and cook until tender, about 10 minutes. Stir in the lemon juice and zest and the crème fraîche and simmer for 2 minutes. Remove from the heat and stir in the sorrel. Season with salt and pepper. Ladle the soup into bowls and serve; pass the croutons at the table.
The croutons can be made a day ahead. Store in an airtight container. The soup can be made without the sorrel and refrigerated overnight. Bring to a simmer and add the coarsely chopped sorrel just before serving.
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