Combine brown sugar, tomato paste, Worcestershire sauce, chili powder, 1 teaspoon salt, and 1/2 teaspoon pepper in a small bowl; rub generously over roast.
Coat a 5-6 qt slow cooker with olive oil. Place red onion slices in a layer in the bottom of cooker, then place roast on top of onions.
Rub potatoes with oil and wrap each potato individually in parchment paper. Arrange on top of roast. Cover and cook on LOW 8 hours or until tender.
Unwrap potatoes; Cut each potato in half crosswise and split each half lengthwise, cutting to but not through the other side. Squeeze each potato half to open for stuffing. Shred roast with 2 forks, and stir to combine with onion mixture.
Top each potato piece with 1/3 cup beef mixture, 1 tablespoon cheese, 1 tablespoon sour cream or yogurt. Then sprinkle with green onions, and salt and pepper to taste.
Note: Each 1/2 potato makes one serving. Depending on how many people you are serving you can save the remaining potatoes and beef for later in the week.