In a large pot melt the butter over medium-low heat. Add the shallots and cook, stirring occasionally, until softened, about 4 minutes. Add the garlic and cook 2 minutes more. Stir in the flour and cook 1 minute more, then add the mushrooms, ¼ cup of the wine, chicken broth, salt and pepper. Bring to a boil, then lower the heat and simmer, uncovered, for 15 minutes, until the mushrooms are tender.
Using a slotted spoon, remove about 2 large ladlefuls of the mushrooms and set aside (don’t worry if there are some shallots mixed in there). Using a handheld immersion blender, purée the soup until smooth. (Alternatively, let the soup cool slightly and use a blender to puree in batches. Be sure to remove the center knob on the blender top and cover with a dish towel to avoid splatters.)
Stir in the cream, the remaining ¼ cup of wine, the thyme, the sage, and the reserved mushrooms. Bring to a simmer, then taste and adjust seasoning with salt and pepper, if necessary. Ladle into bowls and top with the chives and croutons.
Make-Ahead Instructions: This soup will keep nicely in the refrigerator for 3 to 4 days.