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1
Preheat oven to 300u00b0.
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2
Trim and discard excess fat from brisket. In a 12- by 18-in. roasting pan, place onion slices and 3/4 of lemon slices; wrap remaining lemon airtight and chill. Lay brisket on top of onion and lemon in pan.
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3
In a bowl, stir together port, sugar, and marjoram until sugar dissolves, then pour evenly over brisket. Sprinkle brisket with pepper and tightly cover pan with foil.
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4
Bake until brisket is very tender when pierced, about 4 hours. Meanwhile, about 1 1/2 hours before brisket is scheduled to be done, pierce each sweet potato with a fork and set on oven rack alongside or above brisket.
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5
When brisket is tender, uncover pan and return it to the oven to brown the meat slightly, about 15 minutes. Remove brisket from pan and place on a platter; keep warm. When potatoes are soft when pressed, remove them from oven, add to brisket platter, and keep warm.
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6
Skim off and discard fat from pan juices. Place roasting pan on the stove over high heat; boil juices, uncovered, stirring often. As mixture thickens, reduce heat to medium and stir constantly until marmalade is thick and shiny and reduced to 1 1/2 cups, 20 to 30 minutes.
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7
Spoon marmalade over brisket, then garnish with reserved lemon slices. Cut brisket across the grain to serve.
