Peel and mince garlic. Peel and small dice onion. Transfer both to a large bowl.
2 cloves garlic
1 medium yellow onion
Slice mushrooms and add to the bowl.
Peel and slice carrots into thin rounds. Trim and small dice celery. Add both to the bowl.
Preheat a large pot over medium heat.
Once the pot is hot, add butter and swirl to coat the bottom.
¼ cup butter, unsalted
Add veggies to the pot and cook, stirring occasionally, until tender, 6-7 minutes.
Meanwhile, medium dice chicken and season with salt and pepper.
1 ½ lb chicken breasts, boneless skinless
¼ tsp salt
¼ tsp black pepper
Slide thyme leaves off the stems; discard stems.
Add chicken and thyme to the pot and cook, stirring occasionally, 3-4 minutes.
Add flour to the pot and stir to coat the vegetables and chicken. Cook, stirring frequently, about 1 minute.
¼ cup all-purpose flour
Add broth, milk, rice, and salt to the pot. Bring to a boil over high heat, then reduce heat to low and simmer, stirring occasionally, until rice is soft and chicken is cooked through, 15-20 minutes.