Melt butter in a large stockpot over medium heat.
Add the onion, red bell pepper, celery, garlic, and thyme.
Cook until the vegetables are tender.
Add mushrooms to the pot and cook for about 5 minutes.
Stir occasionally until the mushrooms are soft and most of the liquid has evaporated.
In a separate bowl, whisk the milk with the flour.
Slowly stir this mixture into the pot.
Bring it to a simmer and let it thicken for about 15 minutes, stirring often.
Add shredded chicken, artichoke hearts, and peas into the pot.
Pour in the chicken stock and sprinkle onion powder.
Stir everything together and let it simmer until well heated.
Season the soup with salt and pepper to taste.
Serve hot and enjoy with your favorite bread!