In a large saucepan, boil the potatoes until they are tender (about 10 minutes). Mash the potatoes and cream them together with the butter, milk, salt and pepper until they are fluffy. Set aside.
Place all of the sauce ingredients into a blender and blend until smooth. Set aside.
Brown the ground beef and onions in a large skillet or saucepan. Drain the fat off the meat. Add the sauce and chopped vegetables to the skillet. Cover the skillet and allow the meat and veggies to simmer for 10-15 minutes until the veggies start getting tender. Don't allow them to cook all the way through since they will continue to cook in the oven.
Preheat the oven to 350 degrees. Transfer the meat and veggies to a 9x13 pan. Top the meat mixture with the mashed potatoes and spread it out evenly. Bake for 30 minutes at 350 degrees.