Cook bacon until crisp and rough chop. Sauté onion in skillet 2 tablespoons of bacon grease. Cook over medium-low heat until softened, about 5 minutes. Sprinkle onion with 1 tablespoon of flour. Cook and stir 2 minutes. Add vinegar and sugar; stirring until sugar dissolves. Season as desired with salt and pepper. Add cabbages and stir until it slightly wilts, about 1 minute. Add reserved bacon. Remove from heat and set aside. Slice Spam into 6 lengthwise pieces. Place flour, eggs and panko crumbs into three separate bowls. Dip each Spam slice into flour, then egg and then into the panko crumbs. Over medium high heat, sauté the Spam slices in 1/4 cup of vegetable oil until golden brown on both sides. Remove spam, slice bread in half and toast the bread in the hot skillet with a little butter. Spread mustard on both pieces of bread. Add lettuce leaf, spam slice and spoon warm cabbage slaw over top. Cover with second piece of bread. Serve immediately.