This will be the most time-consuming portion of the recipe, but well worth the effort. Great caramelized onions add an irreplaceable naturally sweet, aromatic flavour. The secret to making perfect caramelized onions is no sugar, no butter, and all patience. Slice onions (discarding outer skin and ends) as thin as possible. This will help with speeding up the caramelizing process. Add onions to a large frying pan with 1 Tbsp. extra virgin olive oil and a pinch of salt.
Turn heat to low-medium. It is important to sweat the onions during this first stage (low heat releases their natural juices and softens them.) Stir until soft and translucent. Turn heat up and begin caramelizing. Stir onions frequently to prevent burning. When all of the moisture has cooked off and the pan begins to brown, add 1 Tbsp. of cold water and stir in the onions. This action de-glazes the pan and helps give the onions their brown, caramelized colour. Repeat this process until the onions have an even brown colour. Set aside to cool.
Prepare Honey Dijon Mustard:
In a small mixing bowl, add whole-grain Dijon mustard, honey, and smooth mustard. Stir and set aside.
For best result, gently steam/simmer with small amount of water till heated.
Place wieners on hot, oiled grill to sear for 2 minutes.
Preheat oven (or barbeque) to 375°F.
Spread honey dijon mustard on bun, add beef wiener, 2 slices of cheddar, 2 slices of mozzarella, 1 Tbsp. caramelized onions.
Toast in oven (or barbecue) until cheeses are melted (maximum three minutes).