While the meat is cooking in the oven, peel the carrots, cut them into thin slices and keep them ready in a saucepan with enough water. Chop the parsley very finely. We had small dumplings (finished product from the refrigerated shelf) as a side dish. That is already permitted .... ;-))) especially if you have to process leftover supplies (because of a visit), or have just started to deal with cooking ... because this is where the main thing lies -Attention to the roast. You only have to take into account the time required to cook the dumplings - see their instructions on the packaging, and that is usually around 12-15 minutes. Of course, normal boiled potatoes can also be served.
After the meat has been cooked for the last 15 minutes or more, take the roaster out of the oven and turn it down to 80 °. "Park" the meat briefly on a plate, pour the stock including the onions through a sieve and collect it in a saucepan. Put the onions in an extra bowl, salt them lightly (if they taste delicious, throwing them away would be a shame), put them back in the roaster with the meat and put them back in the oven to keep them warm.
Meanwhile, bring the water for the dumplings to the boil, add salt and let them steep for about 12 minutes. Reduce the temperature a little. You're not supposed to cook. At the same time, add a little salt to the water in the carrots, bring to the boil with the carrots, then reduce the temperature and let them simmer for about 3 minutes. They shouldn't be overcooked until they are soft, but should even be "bite". Then immediately pour off the water, put the pot back on the stove, reduce the heat to a minimum and stir in the butter. When it has completely dissolved, fold in the parsley, keep warm.
Also bring the collected gravy to the boil parallel to the cooking of the dumplings. Try once to see how spicy it is (Kasseler sometimes brings a lot of salt with it from home). Only then decide whether it has to be salted - and, if you like, peppered. When the brew is boiling, stir in the starch mixed with a little water, let it simmer for about 1 minute (the starch must set) and finally stir in the sour cream.
The dumplings should have risen to the surface by now. This is a sign that they have finished cooking. Now take the meat out of the oven, portion it and serve it with the sauce, carrots and dumplings / potatoes. The onions are enough, so everyone can decide for themselves whether they like them.