1. In a medium-sized pot, heat oil. Sauté onion and garlic until fragrant, about three minutes.
2. Remove stems and gills from portobello and cremini mushrooms and slice. Slice button mushrooms. Add all mushrooms to the pot and stir. Sauté over low heat for 10 minutes.
3. Stir in remaining ingredients and bring to a boil. Lower heat and simmer for 45 minutes.
4. Blend with an immersion blender.