Cut and discard extra fat from beef brisket. In an average roasting pan, put onion slices and 3/4 of lemon slices. Wrap rest of the lemon in a plastic and allow it to chill. Put beef brisket on the top of onion and lemon.
Blend brown sugar, port wine or apple juice and marjoram. Stir and let the sugar dissolve. Pour evenly over brisket. Sprinkle pepper on the brisket and cover roasting pan firmly with foil. Bake in a 300 degrees F oven and let the brisket turn tender when pierced. It may take almost 4 hours.
About 1-1/2 hours prior to baking the beef brisket, poke each sweet potato using a fork and place on oven rack alongside or above brisket. When brisket is done, open pan and keep it back in the oven to slightly brown the meat, almost about 15 minutes. Remove beef brisket from pan and put it on a platter to retain its warmth. When sweet potatoes turns tender when pressed, remove them from oven, place it in the brisket platter, and keep it warm.
Skim off and remove fat from pan juices. Put the roasting pan over high flame; boil juices without covering and often stirring. When the mixture concentrates, cut down heat to medium and stir continuously. Cook to make it thick and shiny, and reduced to about 1-1/2 cups in 20-30 minutes. Spoon marmalade over beef brisket and decorate with chilled finely chopped lemon slices. Decorate platter applying kale leaves.
Slice the beef brisket and decorate it across grain.