Pour about a cup of boiling water over dried porcini and let sit for 20 mins or so. Drain liquid and reserve; copped porcini and set aside.
In a large soup pot, heat olive oil over medium heat and sauté onion until translucent. Add chopped mushrooms (including porcini) and sauté until soft. Add flour, stirring quickly to coat. Whisk in broth, porcini liquid, and soy milk. Let simmer, adding water or broth as needed. Puree soup in batches, and season with thyme, salt, and pepper.
This recipe is adapted from Epicurious recipe.