Heat butter in a large soup pot over medium-high heat.
Add onion, carrots, celery, and mushrooms. Cook and stir until onion begins to soften, about 5 minutes.
Add garlic and seasonings and cook just until fragrant.
Add broth, uncooked chicken breasts, and rice. Stir and bring to a boil over medium-high heat.
Reduce heat to medium-low (a simmer), cover, and cook for 20-25 minutes, stirring after 10 minutes.
After 15 minutes remove chicken from the pot and shred.
Once the rice is tender, add chicken back to the pot and serve.
