1/2 C. unsalted butter, divided
3 T. olive oil
1/4 C. diced white onion
8 oz. portabello mushrooms, stemmed and sliced
8 oz. baby bella mushrooms, stemmed and sliced
8 oz. oyster mushrooms, sliced
1-1/2 t. dried tarragon
3 (16 oz) cans chicken broth
1 large baking potato, peeled and diced into small cubes
2 T. fresh thyme
1-1/2 t. dried oregano
1/2 C. heavy whipping cream, optional
salt and ground black pepper, to taste
Heat 1/4 cup butter and oil in a large pot over medium heat until butter melts, 1 to 2 minutes. Add onion; cook and stir until soft, about 5 minutes. Add remaining 1/4 cup butter, portobello mushrooms, baby bella mushrooms, and oyster mushrooms, and tarragon; cook and stir until tender, 5 to 7 minutes.
Stir chicken broth, potato, thyme, and oregano into the pot. Bring to a boil; let soup simmer until potato is tender, 15 to 20 minutes.
Remove soup from heat and puree with an immersion blender until smooth.
Return soup to heat and simmer until heated through, about 5 minutes. Stir in heavy cream. Season with salt and pepper.