Melt butter in a large skillet over medium heat.
Add diced onion and sauté for 5 minutes until translucent.
Stir in minced garlic and cook for 1 minute until fragrant.
Whisk in flour and cook for 2 minutes to create a light roux, stirring constantly.
Gradually pour in chicken broth and heavy cream while whisking to prevent lumps.
Bring to a simmer and stir until the sauce thickens.
Add the cooked chicken, frozen peas and carrots, thyme, salt, and pepper.
Simmer for 5-7 minutes until the vegetables are tender and the chicken is heated through.
Garnish with fresh parsley before serving.