Creamy Mushroom Soup

¼ cup butter
2 T olive oil
2 cups chopped onion
1 cup chopped celery
2 tsp minced garlic
1 ½ tsp chopped fresh thyme
3 (8 oz) packages sliced baby Portobello mushrooms
1/3 cup flour
½ tsp salt
½ tsp ground black pepper
6 cups chicken broth
1 cup heavy whipping cream

In a Dutch oven, heat butter and olive oil over medium heat until butter is melted.  Add onion and celery; cook for 5 minutes.  Add garlic and thyme; cook for 2 minutes.  Add mushrooms; cook for 15 minutes,  stirring frequently.  Add flour, salt, and pepper; cook for 2 minutes, stirring constantly.  Gradually add chicken broth, whisking until smooth.

Bring to a simmer; cook for 20 minutes, stirring occasionally.  Add cream; return to a simmer and cook for 15 minutes.