In a large skillet, brown the meat, half at a time in the butter and oil. Season with salt and pepper. Transfer the meat to a slow cooker.
In the same skillet, brown the mushrooms and onions. Season with salt. Add oil if needed. Add the garlic and cook for about 1 minute. Deglaze with the Marsala wine. Add the stock and thyme. Bring to a boil and pour into the slow cooker. Cover and cook for 4 hours at low temperature. If desired, add the barley and continue cooking for about 1 hour 15 minutes. Remove the thyme sprigs. Adjust seasoning.