Put on a large pot of salted water (Add a large pinch of salt) and bring to a boil. Meanwhile peel and chop potatoes. Once water is boiling add potatoes and boil for 15-25 minutes (Or until potatoes fall apart when poked with fork)
Blend together sauce ingredients and set aside.
Add sauce and filling ingredients to the slow cooker and cook on low 4-6 hours or high 2-3 hours. (Do not stir beef and sauce together until after cooked)
Check potatoes for doneness. (If potatoes fall apart when poked with a fork they are ready to be drained and mashed) If not let boil longer. If potatoes are ready drain and mash together with all mashed potato ingredients. Season with salt and pepper to taste. Set aside to cool.
Once beef/filling is cooked, break apart the beef and mix together filling.
Top off with mashed potatoes and cook on low for 30 minutes.