Add the potatoes, carrot, and onion to a large pot. Cover the vegetables with water then stir in ½ tsp salt and bring to a boil over high heat.
When water begins to boil place a lid on the pot. Reduce the heat to medium and cook until the vegetables are tender, about 10 minutes.
Drain the vegetables in a colander then place the drained vegetables in a large bowl and set aside.
Add the butter and milk to the now empty pot over medium-low heat. Simmer while stirring until the butter has melted and the milk is warmed through, about 2 minutes.
Pour the milk and butter in the bowl with the vegetables then stir in ½ tsp salt and ½ tsp pepper.
Use a potato masher to mash the vegetables well.
Taste and season with additional salt & pepper if necessary.
