Bring a large frying pan to a medium-high heat. Once heated, add sliced onions, oil, and water. Mix lightly and cover. Allow to steam for 10 minutes. Note: adding water helps speedup the caramelizing process.
Remove the lid and continue frying until the onions become golden brown and caramlized. Set aside. Note: add additional oil if needed.
Meanwhile, bring a large frying pan to a medium-high heat and add olive oil and beef mince. Fry until most of moisture has escaped and the mince starts to brown.
Add the seasonings and tomato paste. Mix and fry until the tomato paste starts to darken, approx 3 minutes. Then add water, soy sauce and processed cheese. Mix, cook for 30 seconds, remove from the heat and set aside.
In a small bowl, add mayonnaise, tomato sauce, yellow mustard, dice gherkins and mix until combined.
Take two slices of bread and layer each one with some of the burger sauce, top with 2-3 slices of processed cheese, savoury mince, caramelized onions. To the remaining two slices, add more sauce and cheese each slice and place that on top of the loaded slices.
Bring a large frying pan to a medium-low heat, add a dash of oil and butter, once melted place the 2 loaded toasties in the pan, ensuring they get and equal amount of butter and oil. Toast on each side for 2 minutes.
Once toasted on both sides, remove from the heat, cut in half and serve immediately.