In large skillet, melt 1 tablespoon butter over medium heat.

Add onion, garlic, and carrots; cook and stir until tender, about 7 minutes.

Remove from heat and place in medium bowl; let cool for 15 minutes.

Meanwhile, cut round steak into 4 equal portions.

Trim excess fat, then place between sheets of waxed paper and pound until ¼-inch thick.

Divide onion mixture on beef. Roll up, enclosing filling; secure with toothpicks.

On plate, combine flour, salt, and pepper and mix.

Dredge beef rolls in flour mixture. In same skillet, heat vegetable oil.

Brown beef rolls on all sides.

Place beef rolls in 4-quart slow cooker.

Deglaze skillet with Stock; add tomato sauce, mustard, and Worcestershire sauce and bring to a simmer.

Pour into slow cooker.

Cover and cook on low for 7–8 hours or until beef is tender.