How to Make It
In a large pot, heat the oil and butter over low heat. Add the onions and garlic. Cover and cook, stirring occasionally, until very soft and beginning to turn golden, about 30 minutes. Raise the heat to moderate and continue cooking the onions and garlic, uncovered, stirring frequently, until deep golden, about 10 minutes longer.
Add the stock, bread, bouquet garni, and salt. Bring to a boil. Reduce the heat and simmer about 15 minutes.
Remove the bouquet garni and puree the soup in a blender or food processor. Strain the soup back into the pot. Add the half-and-half and pepper and bring back to a boil. Serve topped with the croutons, if you like.