Step 1

Stir together the mustard, sugar, and garlic in a small bowl, forming a paste; set aside.

Step 2

Heat the oil in a large nonstick skillet over medium-high. Sprinkle the beef with the salt and pepper. (If needed, cut the beef into 2 pieces to fit in the skillet and slow cooker.) Add the beef to the hot skillet; cook until browned, about 4 minutes on each side. Transfer the beef to a 5- to 6-quart slow cooker, reserving the drippings in the skillet. Spread the paste over the brisket.

Step 3

Add the onions to the reserved drippings in the skillet; cook, turning once, until the onions are lightly browned, about 4 minutes. Place the onions on the top of the beef in the slow cooker, reserving the drippings in the skillet. Whisk together the beef stock and tomato paste in a medium bowl; add the stock mixture to the reserved drippings in the skillet, and cook, stirring to loosen the browned bits from the bottom of the skillet, about 30 seconds. Pour the stock mixture over the onions and beef in the slow cooker. Add the rosemary sprig. Cover and cook on LOW until very tender and the beef is beginning to fall apart, about 8 hours. Transfer the beef to a cutting board, reserving the cooking liquid in the slow cooker. Discard the rosemary sprig. Let the beef rest 10 minutes.

Step 4

Meanwhile, prepare the potatoes according to the package directions. Transfer to a serving bowl, and stir in the 2 tablespoons butter until melted.

Step 5

Thinly slice or coarsely shred the beef. Serve the beef and onions over the mashed potatoes; sprinkle with the parsley, and serve with the reserved cooking liquid.