Preheat a large soup pot (I used a 5.5 quart) on medium high heat. Add remaining 2 Tablespoons of olive oil. Saute onions in olive oil for 10 minutes, until they are tender and translucent and starting to brown. Add in garlic, cook for 2 more minutes until fragrant. Add the rest of the ingredients and the reserved mushrooms. Bring to a boil and lower to a simmer. Partially cover and cook over low heat for at least 30 minutes, or up to an hour for richer flavor. Serve with desired accompaniments.