Place beef cut into slow cooker on the bed of onions. Pour chicken broth, soy sauce, balsamic vinegar, salt, pepper, garlic powder and brown sugar evenly over meat.
Set the pats of butter across the top round in a line.
Cook on high for at least 4 hours or low for at least 6 hours, or until the temperature reaches 140 degrees internally. Remove meat from slow cooker and place into mixing bowl; shred with forks.
Meanwhile, peel and slice potatoes into chunks. Boil until you can mash them with a fork, then mash with sour cream, milk, rosemary and garlic powder.
When the meat is done cooking, melt the two tablespoons of butter in a sauce pan. Whisk in the flour until it is smooth, then scoop drippings from the slow cooker and allow it all to simmer on low until it thickens into a gravy. You can add dried rosemary at this stage to taste for some extra flavor.
We like to serve in a bowl with mashed potatoes on the bottom, shredded meat and onions on top, and drizzled with our homemade gravy.