Slice the butter thinly and place it in the bottom of the slow cooker insert.
Wash the potatoes and add them on top of the butter in the slow cooker.
Peel and cut the carrots. Peel and cut the onion. Layer the carrots and onion on top of the potatoes.
Place the sirloin tip roast on top of the potatoes, carrots and onions.
In a microwave safe bowl or glass measuring cup, whisk together the beef stock, apple juice, tomato juice, Worcestershire sauce and corn starch. Stir in the rest of the ingredients, onion flakes through pepper.
Microwave the beef stock mixture to dissolve the corn starch, about 90 seconds.
Pour the beef stock mixture over the roast and vegetables.
Place the lid on and cook on LOW heat for 8 - 10 hours, until the beef roast is fall-apart tender.