Place cubed potatoes in a large saucepan and cover potatoes with cold water. Add 1 teaspoon kosher salt and set saucepan over high heat.
Bring to a boil and cook until potatoes are tender when pierced with a fork, about 15 minutes.
Drain potatoes, return to saucepan and set over medium heat, stirring the potatoes until dry, 1 to 2 minutes. Remove from the heat and cover to keep potatoes warm.
In a medium saucepan set over medium heat, combine milk, butter, remaining salt and freshly ground black pepper. Stir occasionally until butter is melted.
Reduce heat to low.
Rest a food mill, fitted with the medium disc, on top of the saucepan with the milk mixture.
Working with about one-third of the potatoes at a time, run the potatoes through the food mill, directly into the saucepan. Alternatively, use a potato ricer.
Using a rubber spatula (a heatproof one, please) stir the potatoes into the milk mixture until completely combined.
Stir the caramelized onions and bacon into the mashed potatoes. Taste and add additional salt and pepper, if necessary. Serve.