Traditional schnitzel sides are sauerkraut, which everyone loves, and a German Potato Salad, but I always serve it with mashed potatoes because everyone in my family loves mashed potatoes. I made a big batch so there are plenty of leftovers for tomorrow. My mashed potatoes have a make-ahead advantage—you can get those potatoes boiling way in advance and then mash the potatoes just before eating—or mash them all the way, then reheat with a little half-and-half right as the schnitzel finishes, so there's not a lot of last-minute juggling at the stove top. This way, you can put all your focus into mastering the schnitzel itself, and still sit down to a hot dinner.
Add the potatoes to a large pot with enough water to cover them. Season generously with salt so that it tastes like sea water (this is important because you're cooking the potatoes skin-on, which keeps the creamy potato flavor locked in, but doesn't allow all the salt to penetrate).
Bring to a boil and cook until creamy, about 40 minutes. Drain and return the potatoes to the pot.
Use a clean kitchen towel to hold the potatoes and gently peel off the skin and discard.
Add the potato flesh to a colander set in a large bowl and use a spatula to press the potato flesh through. (If you have a food mill or a ricer, it works much more quickly, but this also does the trick without requiring you to store an extra tool.)
Stir in the butter to coat the potatoes completely in the fat first. Meanwhile, heat the cream and milk in a small saucepan until simmering. Gently mash into the potatoes with a potato masher or a wooden spoon, adding in the sour cream just before serving and seasoning more with salt and pepper as needed.
Serve hot. Store any leftovers in an airtight container with a lid in the refrigerator for up to 2 days, and reheat with additional cream, half-and-half or milk before serving.