In a mixing bowl, combine ¼ cup oil and mushrooms. Toss to coat mushrooms with oil. Transfer to a baking sheet and bake in a 400°F degree oven for 20 minutes. Remove mushrooms from oven and cool. Chop or grind mushrooms. Set aside.
Heat remaining oil in a large sauce pot. Add onions and garlic. Cook and stir for 5 minutes until soft.
Add remaining ingredients. Blend ingredients to desired consistency with an immersion blender. Bring sauce to a boil, turn heat to low and simmer for 15 minutes. Cool according to accepted cooling standards.
