Soak mushrooms in hot water for at least 10 minutes. Drain, reserving liquid, and chop mushrooms.

Heat oil and butter in a large pot. Sauté onion and garlic until soft, about 3-5 minutes.

Add chicken broth, thyme, chopped mushrooms, and 2 cups reserved mushroom liquid. Simmer half-covered for 30 minutes.

Blend about 3/4 of the soup until smooth, then return to pot.

Stir in heavy cream, salt, and pepper. Adjust thickness with more mushroom liquid if needed.