Preheat the oven to 400°F. Line a baking sheet with parchment paper or cooking spray.
Heat the oil for the caramelized onions in a large cast iron pan or skillet. Heat on high until one line of smoke appears. Add the onions and lower heat to medium-high. Cook the onions until caramelized and a dark brown color starts to develop on bottom of pan, stirring occasionally. Season with salt, reduce the heat and continue to cook until the onions are sweet and melted, about 25-30 minutes total.
In a mixing bowl, break the bread into small pieces. Add the milk and combine into a thick paste. Add the remaining meatball ingredients and combine with your hands, being careful not to overmix. Roll the mixture into 2-inch meatballs and place on the prepared baking sheet. Bake for 30 minutes, or until cooked through.
Meanwhile, in a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth.
For the cherry gastrique, bring all of the ingredients to a simmer in a saucepan. Cook until the cherries are soft. Purée in a blender or with an immersion blender until smooth. Adjust the seasoning to taste.
Bring a large pot of salted water to a boil for the pasta. Add the pasta, stirring to prevent clumps. Cook to al dente, according to package instructions, strain and return to the pot, drizzled with oil to prevent sticking. Add the caramelized onions and toss to combine.
To serve, divide the pasta between plates. Top with mashed potatoes, meatballs, cherry gastrique, sauerkraut, scallions and sour cream.