Meanwhile, finely mince the onion and sauté it in a little butter until softened and translucent. Add the onion into the batter.
Heat a pancake pan until very hot. Generously butter the pan and pour a first batch of the batter in the rounds. Fry until the bubbles on the batter surface begin to set and flip the pancakes over. Continue frying with the rest of the batter. Serve the blood pancakes hot, accompanied bylingonberry jam or sugared lingonberries.