Peel and quarter the potatoes. Peel and coarsely shred (grate) the carrots. Peel and dice the onion.
In a large saucepan, cover the potatoes and carrots with 2" (5cm) of cold water and bring to a boil over a high heat. Reduce the heat and simmer until the potatoes are tender (about 15-20 minutes).
When the potatoes are fork-tender, drain them and transfer them to a large bowl. Add the milk, butter, and salt and mash with a potato masher until the mixture is smooth (there should be small flecks of carrot visible). Stir in the onions then crumble the bacon over the top. Serve immediately.