Add the olive oil to a pot over medium heat. Add the onions, garlic, and mushrooms, and saute for about 3 minutes. Add the remaining ingredients and simmer for 20 - 30 minutes.Blend the soup by using an immersion blender or transfering it to a regular blender. (If using a regular blender, be very careful as the steam can sometimes cause the lid to blow off. I recommend allowing the soup to cool for a few minutes, then blending small batches on medium speed while holding the lid down with a large pot holder or towel in my hand).Add freshly-ground black pepper and sea salt to taste. Garnish with some crushed pepper, fresh herbs, or a drizzle of olive oil. Serve hot.