In a small bowl soak mushrooms in enough hot water to cover for 30 minutes. Rinse well and squeeze to drain thoroughly. Thinly slice mushrooms, discarding stems.
In a large saucepan combine mushrooms, broth, sliced carrot, dried onion, lemon juice, dried basil (if using), and garlic powder. Bring to boiling. Reduce heat and simmer, covered, for 8 to 10 minutes or until carrot is almost tender, stirring occasionally.
Stir in fresh basil (if using) and spinach. Simmer, covered, about 2 minutes more or until spinach wilts. Makes 8 servings.