In a 6 quart pot, melt butter over medium heat.
Add onion, celery, carrots, mushrooms, garlic, and red pepper flakes.
Cook, stirring frequently, until vegetables are tender.
Add chicken, rice, tomatoes, and stock.
Bring to a boil, then lower heat.
Cook over low to medium low heat for 30 minutes, stirring occasionally, until rice is cooked.
Season to taste with salt and pepper.