Turn the slow cooker on to the warm setting. Evenly distribute the salt on the chicken.
In a large skillet, warm the oil and butter over medium-high heat. Brown the chicken, about 2 1/2 minutes per side. Place the chicken into the slow cooker and cover.
Reduce heat to medium and add the shallots. Cook for 1 minute. Add the marsala, white wine, vinegar thyme and bay leaf to the skillet. Deglaze the pan and bring to a simmer.
Once simmering, add the chicken stock, Dijon, ground mustard and sugar. Stir well and bring to a simmer. Pour the liquid into the slow cooker. Rearrange the chicken as needed to have all the pieces slightly covered with liquid. Cover and cook on high for 2 1/2 to 3 hours or low for 6 hours.
Remove the chicken, place on a platter and cover. Whisk the flour into the slow cooker and stir. Cook on high, uncovered for 15 to 30 minutes. Taste the Serve the chicken topped with the sauce. Mashed potatoes, crusty bread and are wonderful to serve with the chicken.