You're going to have a few pots going at once for this one, so do your prep work first.
Put your apple slices, turnips and cubed potatoes into separate saucepans. Cover each with water and set the burner to medium heat.
While all these are cooking, fry up your bacon. Once the bacon is cooked to your satisfaction, remove from heat and place on a paper towel covered plate.
Fry your sliced onions in the bacon grease until softened, turn the heat off. (For non-bacon eaters, omit all mention of bacon or use a bacon replacement at will!)
Monitor your other three pans, remove the potatoes, turnips and apples when they're tender. Combine your potatoes, turnips, apples and onions in a single pot, mash them with a potato masher (or gently use your food processor on them). Stir in some butter (or butter replacement), salt and pepper. Top with bacon, serve, enjoy!