Combine brown sugar, tomato paste, Worcestershire sauce, chili powder, 1 teaspoon salt, and 3/4 teaspoon pepper in a small bowl; rub generously over roast.
Place red onion slices in bottom of a 5- to 6-quart slow cooker coated with cooking spray; top with roast.
Rub potatoes with oil, remaining 1/4 teaspoon salt, and 1/4 teaspoon pepper. Wrap each potato in parchment paper; arrange on top of roast. Cover and cook on LOW 8 hours or until tender.
Unwrap potatoes; split lengthwise, cutting to but not through the other side. Shred roast with 2 forks, and stir to combine with onion mixture.
Top each potato with 1/2 cup beef mixture, 2 tablespoons cheese, 1 tablespoon sour cream, 2 teaspoons green onions, and 1/4 teaspoon pepper.
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