In a large, nonstick saucepan, heat olive oil over medium-high heat. Add onion and cook until softened, about 5 minutes. Stir in garlic; cook 30 seconds. Add mushrooms and carrots, and cook 5 minutes more.
Stir in remaining ingredients plus 1 cup water. Bring to a boil; reduce heat to medium-low, cover, and simmer 20 minutes. Transfer to food processor, and pulse to puree.