Start by caramelizing the onions as they will take the longest. Slice the onion and add it to a pan over medium heat with the butter and olive oil. Sprinkle the sea salt on the onions.
Allow the onions to cook down and start to brown for 10 minutes, stirring couple of minutes. After 10 minutes of caramelizing, add the water to the onions and cook for an additional 5 minutes until the water has been completely absorbed. Set the caramelized onions aside.
Slice your bread and freshly grate the 4 cheeses. I recommend not skipping this step as fresh grated cheese melts better than pre grated bagged cheese.
Combine the softened butter, truffle oil, and garlic mixing to combine. Brush the butter on one side of each slice of bread.
In a pan over medium high heat, add one slice of bread truffle butter side down. Sprinkle the cheese and onions on top before adding the second slice of bread, butter side up.
Allow the grilled cheese to cook for 1-2 minutes on the first side before flipping. You can use a spatula or heavy pan to press the sandwich down to get it nice and crispy.
Remove from heat, slice, and enjoy with a dirty martini, just how Serena would order it!