Preheat your oven to 350F. Spread the red onion marmalade evenly on two slices of brioche. Mound the cheese over the red onion marmalade. Place the other slice of brioche on top of the cheese. Heat an All-Clad nonstick skillet over medium heat. Add the clarified butter. Carefully set your sandwiches in the hot pan. Cook gently until the brioche is golden brown. Turn the sandwiches over and place in the oven. Bake for about 8-10 minutes or until the cheese is melted and the bottom slice of brioche is browned. Slice and serve with a green salad, tomato soup or pickles.
Place all ingredients except for the vinegar in an All-Clad 4qt sauce pot. Bring to a gentle boil. Cook the onions down until they are very soft and all of the liquid has been evaporated. Add the vinegar and cook down until all the liquid is evaporated. Transfer to heat resistant container and chill. Reserve in the refrigerator in a sealed container for up to a week until needed.