Gently fry the bacon slices in a heavy saute pan. Remove and drain on paper towels. Spill out all but 1 tablespoon of the rendered bacon fat, turn heat to high and add the sliced onions to the pan. Cook over high heat for 2 minutes, or until the onions just begin to turn brown. Remove onions and drain on paper towels.
Spread 1 teaspoon of butter on each of the bread halves. Top each half with a lettuce leaf that overlaps slightly on all sides. Spread 1 ounce of liverwurst across each lettuce leaf. Cut the bacon slices in half, and top each smorrebrod with a half-slice of bacon. Divide the Sauteed onion rings evenly among the 12 smorrebrod, and garnish each one with the parsley.