Pour 1 cup boiling water or broth over the porcinis.  Let soak until mushrooms soften, about 20 minutes. Drain, reserving broth but discarding any sediment in broth. Coarsely chop porcini mushrooms.

Heat olive oil in heavy large pot over medium-high heat. Add chopped onions and sauté until tender, about 7 minutes. Add mushrooms and sauté until brown and tender, about 8 minutes. Add minced garlic, thyme, and porcini and sauté 2 minutes. Sprinkle flour over; stir 1 minute. Gradually whisk in reserved porcini soaking liquid and milk. Bring to boil, stirring frequently. Reduce heat to medium and boil gently until soup thickens slightly, about 12 minutes.  Use immersion blender to puree until smooth. Season soup to taste with salt and pepper. Ladle soup into bowls and serve.