In a skillet, heat 1 tablespoon of the 2 tablespoons olive oil and cook 1 cup onion, 1 cup celery, 1 cup carrots until softened, add the 3 cloves garlicand cook 30 seconds.
Place the vegetables in the bowl of a slow cooker.
For the chicken
Trim 8 boneless skinless chicken thighs , cut into smaller pieces.
Season the chicken with 1 teaspoon salt and 1/2 teaspoon pepper, then dust with 1 tablespoon all-purpose flour.
Heat remaining 2 tablespoons olive oil in the skillet over medium-high heat, cook thighs until browned on both sides.
Transfer to the slow cooker
For the sauce
Pour 4 cups chicken stock into the skillet and scrape up all the browned bits from the bottom.
Bring to a boil, then whisk in the 1 tablespoon Dijon mustard and 1 teaspoon dried thyme.
Cook
Pour the sauce over the ingredients in the slow cooker. Mix well.
Cook on low for 4 hours.
Finish
In a bowl, whisk together1/2 cup sour cream and 1 teaspoon cornstarch.
Mix into the slow cooker and cook 30 minutes more.