Heat oil in a large saucepan set over medium heat. Add mushrooms and sauté until tender; remove from pan and reserve. In the same pan, heat butter over medium. Add onion and sauté until softened, then add garlic paste.
Sprinkle flour over vegetables and stir until the flour mixture forms a paste. Gradually stir in broth until flour dissolves. Bring to a simmer, stirring constantly, the flour mixture forms a paste. Add half of the reserved mushrooms and their accumulated juices. Reduce heat to low and simmer for 10 min.
Using a hand-held or stand blender, purée soup until smooth (fill a stand blender only halfway and hold lid in place with a tea towel while blending).
Stir in remaining mushrooms, salt, pepper and thyme, and bring soup up to a simmer. Remove from heat, stir in cream, garnish with chives, top with croutons and serve.
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