Stir in onions and cook 1 minute. Heat to a boil. Add remaining ingredients. Add additional stock or broth if vegetables aren't covered with liquid. Cover casserole and transfer to oven.
Transfer roast to a serving platter and arrange vegetables around it.
Spoon some of the sauce over and pass the remaining sauce separately. The chocolate buttercream is optional. Heat the oven to 350 degrees. Tap out the excess flour. Stir constantly until chocolate is melted and smooth.