Saute 1 chopped onion in 1 tablespoon butter. Transfer the onion to a bowl. Add the ground meat, grated garlic cloves, breadcrumbs, egg, mustard, thyme or marjoram, ¾ teaspoon salt, and some pepper to the onions in the bowl. Mix well with your hand. Form 15 meatballs. Wet your hands a few times in between, it will be much easier to form the balls.
Pour the stock/broth in a larger saucepan or a deeper skillet. Bring to a boil, turn the heat down, add the meatballs, cover, and simmer for 15 minutes or until the meatballs are cooked through. Remove the meatballs with a slotted spoon and keep them warm. Strain the liquid into another pot.
Wipe the saucepan or the skillet with kitchen paper. Melt the remaining tablespoon butter and fry the second chopped onion for about 3 minutes.
In the meantime clean the mushrooms with kitchen paper and slice them. Add them to the pan and continue cooking for 3 minutes.
Add about ½ cup of the reserved meatball broth, cover, and cook for about 5 minutes on low heat, until the stock reduces. Add the remaining stock and bring to a boil.
In a small bowl mix together the corn starch and a little water. Add the mixture to the gravy while whisking all the time to prevent the formation of clumps. Simmer for a couple of minutes until the sauce thickens slightly. If too thick, add a bit more water or broth.
Whisk in the sour cream, adjust the taste with salt and pepper and add the meatballs to the pot as well. Cover and heat through.
Sprinkle with chopped chives and serve with boiled potatoes or noodles and with gherkins or other pickled vegetables.