Preheat the oven to 350 degrees F (175 degrees C).
Spread Dijon mustard over the brisket.
Arrange onions in the bottom of a Dutch oven. Top with sweet potatoes and baby potatoes. Place brisket over the potatoes. Sprinkle salt evenly over brisket and potatoes.
Stir maple syrup and soy sauce together in a small bowl. Pour mixture over the beef. Cover Dutch oven.
Bake in the preheated oven until beef is tender and easy to slice, about 2 1/2 hours. Let cool to room temperature, at least 30 minutes. Refrigerate 8 hours to overnight.
Preheat the oven to 250 degrees F (120 degrees C).
Slice beef and return to the Dutch oven. Bake until warmed through, 30 to 45 minutes.
