In your slow cooker, set it to the sauté function and add vegetable oil and unsalted butter. Heat for about 2 minutes, then add chopped onion and minced garlic. Sauté for 3–5 minutes until translucent.

Sprinkle plain flour, salt, black pepper, dried thyme, and celery salt into the mixture. Stir for 1 minute, then gradually pour in chicken stock and lemon juice. Whisk continuously for about 2 minutes until smooth.

Place boneless chicken thighs, mushrooms, chantenay carrots, and diced celery into the slow cooker. Gently fold to coat chicken and vegetables in the sauce.

Cover with the lid and cook on low for 6–8 hours or high for 4–5 hours until chicken is tender.

30 minutes before cooking ends, stir in the double cream and simmer without the lid to thicken the sauce.

Once cooked, adjust seasoning if needed and serve portions into bowls, garnishing with fresh parsley. Pair with mashed potatoes or bread.