Heat butter and olive oil in large Dutch oven over medium heat. Add sliced mushrooms and cook undisturbed for 4-5 minutes until golden brown on one side. Stir and cook another 3-4 minutes until tender.
Add diced onion, carrots, and celery to pot. Cook for 5-6 minutes, stirring occasionally, until vegetables soften. Add minced garlic and cook for 1 minute more until fragrant.
Sprinkle flour evenly over vegetables and mushrooms. Stir constantly for 2 minutes until flour is absorbed and no white powder remains.
Slowly pour in chicken broth while stirring constantly to prevent lumps. Add dried thyme and bay leaf. Mix until evenly combined.
Bring mixture to gentle boil, then reduce heat to low and simmer uncovered for 10-12 minutes until vegetables are completely tender.
Stir in shredded rotisserie chicken and simmer for 5 more minutes to warm through and absorb flavors.
Reduce heat to lowest setting. Pour in heavy cream slowly while stirring gently. Keep at very gentle simmer to prevent cream from curdling.
Remove bay leaf. Season with salt and pepper to taste. Ladle into bowls and garnish with fresh parsley. Serve hot.
Notes
Use both white and dark meat from rotisserie chicken for best flavor.
Don’t stir mushrooms immediately after adding – let them caramelize first.
Mix mushroom varieties for more complex flavor.
Freeze soup base without cream for up to 2 months, add cream after reheating.
Rotisserie chicken is already seasoned, so taste before adding salt.