For the creme fraiche:
1/3 cup sour cream
1/3 cup heavy cream
For the chicken:
1 tablespoon butter
1 (10-ounce) package sliced mushrooms
2 leeks, halved lengthwise, thinly sliced and washed
2 tablespoons white wine
1 tablespoon Dijon mustard
1 teaspoon thyme
1/2 teaspoon celery salt
1 teaspoon black pepper
2 pounds boneless, skinless chicken thighs, trimmed
In a glass jar, mix together the sour cream and the heavy cream. When the creams are evenly blended, cover the jar and let it sit at room temperature. It will be used just prior to serving.
Grease the slow cooker crock with the butter, leaving the excess in the crock. Add all ingredients to the slow cooker except for the chicken and creme fraiche and mix well.
Add chicken and mix everything together. Cover and cook on low for 6 hours.
When done cooking, use tongs to transfer the chicken to a plate. Cover to keep warm. Remove 1/4 cup of the cooking liquid and discard.
Stir the creme fraiche into the leek and mushroom mixture, then return the chicken to the cooker and stir gently to coat with the sauce.
Crock-Pot 7-Quart Oval Manual Slow Cooker, 7 Quart