Place everything in a slow cooker and cook on low for 8 hours with the lid on and another 2-4 hours with the lid ajar, about a 1 inch opening to let the moisture get out. Note: Cook the onions in the slow cooker until a deep golden brown and most of the liquid has cooked off.Enjoy right away or store in a seal container in the fridge for up to a week.Butter one side of each slice of bread and assemble the sandwiches with the cheese, roast beef onion jam, arugula and the mixture of the mayo and the horseradish between slices of bread with the buttered sides on the outside.Grill in a pan over medium heat until lightly golden brown on the outside, about 2-4 minutes per side.Place the onions in a large oiled baking dish with the tops and bottoms facing up, pour the mixture of the remaining ingredients over the onions and cover in foil.Roast in a preheated 400F/200C oven for 40 minutes before removing the foil, gently flipping the onions and roasting until tender, about 20 minutes.Serve topped with the balsamic glaze from the bottom of the pan.