How to Make It
Melt 3 tablespoons of the butter in a large saucepan. Add the garlic and cook over low heat until softened, about 10 minutes. Sprinkle the flour over the garlic and cook, stirring, until golden, about 4 minutes. Raise the heat to moderate and gradually add the water, stirring constantly, until it boils. Reduce the heat to moderately low, cover and simmer the soup until the garlic is very soft and falling apart, about 30 minutes.
In a large skillet, melt the remaining 4 tablespoons of butter over moderate heat. Add the bread and cook, stirring occasionally, until crisp and golden brown, about 10 minutes. Drain the croutons on paper towels.
In a medium bowl, whisk the egg yolks with the vinegar. Gradually whisk 1 cup of the hot soup into the eggs. Reduce the heat to low. Add the eggs to the saucepan, stirring constantly until slightly thickened, about 5 minutes; do not let the soup boil or it will curdle. Season with salt and pepper and serve hot, with the croutons.