Drain the cooked sweet potatoes and place in a food processor...this is a little trick to make the sweet potatoes have an extra creamy mouth-feel with out lots of fat...it gives you the feeling like you are having a really rich cheesy topping! Mix in cheese and allowed butter and process for a minute or two until silky smooth (or mash it all together by hand if you want to forgo this step).
Mash the yellow potatoes with a little butter and milk by HAND, using a hand masher. Unlike sweet potatoes, you can not process these or you will have some very gummy mash on your hands! You can mix in the cheese here too! Mash with enough preferred milk to create a smooth and creamy mashed potato.
Season with sea salt, ground pepper. Add a couple of tablespoons of any milk only as necessary to achieve the creamy mashed texture.
Layer the beef and veggie mixture in a casserole or baking dish.
Recipe via tessadomesticdiva.com