Place dried mushrooms in bowl; add boiling water. Let stand until mushrooms are softened, about 15 minutes. Drain, reserving liquid. Slice mushrooms, discarding tough stems from black mushrooms.
In a lightly greased saucepan, saute onion until tender, about 5 minutes. Add dried mushrooms, reserved liquid and stock; heat to boiling. Reduce heat and simmer, covered 20 minutes, adding cremini mushrooms during last 10 minutes.