We have loads of lovely corned beef left over, something I planned for as soon as I saw . Head on over there if you're planning to have some leftover corned beef; they have .
The first thing I had to do to make these was cook the onions. I sliced 1 medium onion thinly, then sauteed in butter over medium heat for about 20 to 30 minutes, stirring occasionally. When they were softened and beginning to caramelize, I seasoned with salt and balsamic vinegar. In the meantime, I sliced corned beef and fontina cheese. I spread softened butter on one side of slices of pumpernickle bread (the buttered side is the outside of the bread), and mustard on the other side of the slices. I piled on the cheese, corned beef and cooked onions, then put the sandwiches in a clean skillet. Turned the heat to medium-high and cooked until the bread was toasted and the cheese melty, flipping the sandwiches once to toast both sides.