Finely chop the peeled onion. Fry in oil, add the washed and dried liver, fry together for about 7 minutes.
Cool slightly, place in a blender, and blend. You can also use a meat grinder, but then the mixture will not be like a pâté, but will contain small particles of liver.
Add the softened butter and salt, and blend again with a blender.
Transfer the mixture to a container, cover with a lid, and refrigerate for 1.5 hours.