Cook the barley in salted water until soft, drain, and let cool.
Chop the onion and sauté in butter until soft. Let cool.
Making Mustamakkara at home is time-consuming and requires specific ingredients. Most Finns buy it ready-made, fresh, and already pre-cooked, which they then simply reheat at home.
This recipe is simplified for preparing the 'filling' that is stuffed into casings. The key to its flavor is the combination of blood, barley, and fat. The most important part is knowing how to heat and serve it correctly.
Cook the barley in salted water until soft, drain, and let cool.
Chop the onion and sauté in butter until soft. Let cool.
In a large bowl, mix the pork blood, cooked barley, ground meat, and fat. Add the sautéed onion, salt, black pepper, and marjoram.
Mix the mixture thoroughly until homogeneous. If necessary, add a little broth so the mixture is not too dry.
Stuff the mixture into the sausage casings (using a stuffer or funnel). Do not stuff too tightly, as the barley will expand. Tie the ends.
Submerge the sausages in hot water (not boiling, about 80 °C (176 °F)) and boil for about 60-90 minutes until cooked through and firm. They should be pre-cooked but will not have a crust.
The pre-cooked sausages are traditionally reheated in a pan with butter, or they can be briefly baked/fried to gain a crispy and dark crust.
Slice the warm Mustamakkara into thick rounds. Serve immediately with a generous spoonful of lingonberry jam (Puolukkahillo) and a cold glass of milk. This is the authentic Finnish way to consume it.