Melt butter in a large skillet over medium-high heat.
Add chicken and cook until browned and opaque throughout, about 5 to 7 minutes.
Add onion, garlic, and frozen vegetables; sauté for 3 minutes until onions are translucent.
Sprinkle flour over the mixture and stir for 1 minute to incorporate and cook out the raw flour taste.
Slowly whisk in chicken broth and heavy cream, scraping the bottom of the pan to release browned bits.
Stir in thyme, salt, and pepper.
Simmer for 3 to 5 minutes until the sauce thickens to a creamy consistency.
Remove from heat and serve immediately in shallow bowls.