Cook the pasta together with the stock cubes according to the instructions on the packet.
Meanwhile, sweat a finely chopped onion in melted butter in a small saucepan. Add the minced meat and fry until crumbly. Fry well so that it is brown. Add 1-2 tablespoons of the pasta water to the meat. Now add the ketchup and some cream - possibly also some mustard. Season to taste with salt and pepper. Add the sauce thickener and cook for about 1-2 minutes.
Now pour off the pasta water and mix the minced meat sauce with the pasta. Serve hot.
Variation: Sometimes in Denmark the mince is extinguished with white wine and mostly the Madkulør (brown food color) typical in DK is added. You can also add a few carrots or a bell pepper.
My family has this dish instead of pasta Bolognese - about once or twice a week. After all, my mother is also Danish.