Lightly coat the inside of a slow cooker with cooking spray.
Combine the Dijon mustard, garlic oil, tarragon, salt, and pepper in a small bowl. Place chicken thighs in the bottom of the slow cooker. Spread half of the Dijon mixture on top. Flip chicken thighs over and spread the remaining Dijon mixture on top. Carefully pour the wine in around the chicken thighs.
Cook on low 5 hours or high 2.5 hours.
Remove chicken to a plate and cover to keep warm.
Pour 1 cup of the liquid remaining in the slow cooker into a small saucepan. Bring to a simmer over medium-high heat.
In a glass measuring cup or small bowl, whisk together the heavy cream and cornstarch. Slowly pour into the saucepan while stirring. Continue to cook and stir until thickened. Stir in the lemon juice.
Serve the sauce poured over the chicken thighs and garnished with parsley if desired. This is delicious served over steamed rice.
Notes
Depending on how many people you are feeding with this recipe, each serving contains 1 tablespoon or less of heavy cream, which should be tolerable to most people with IBS. (You can read more about cream and FODMAPs here: https://www.fodmapeveryday.com/all-about-cream-fodmaps/) If you would rather not use heavy cream, 3 tablespoons lactose-free whole milk and 1 tablespoon butter would be a good substitute to get the same richness. Simply whisk the cornstarch into the milk and add the butter with the lemon juice, stirring until melted.