Melt butter in a large skillet or Dutch oven over medium heat.
Add onion, carrots, and celery; sauté for 5 minutes until softened.
Stir in garlic and chicken pieces, cooking until chicken is browned on all sides, about 4 minutes.
Sprinkle flour over the mixture and stir constantly for 1 minute to cook out the raw flour taste.
Slowly whisk in chicken broth to avoid lumps, then bring to a simmer.
Stir in the dry orzo, thyme, salt, and pepper.
Reduce heat to medium-low, cover, and simmer for 10-12 minutes or until orzo is tender and most liquid is absorbed.
Stir in heavy cream and frozen peas; cook for an additional 2 minutes until heated through and sauce is thickened.
Adjust seasoning to taste and serve immediately.