Heat the extra virgin olive oil in a skillet over medium heat and add the thinly sliced yellow onion.
Cook the onions for 10-12 minutes, stirring occasionally, until they are softened and golden brown.
Stir in the balsamic vinegar to the onions and cook for 1 more minute to glaze, then remove from the pan and set aside.
Season the thinly sliced sirloin steak strips with sea salt and black pepper.
In the same skillet over medium-high heat, sear the steak strips for 2-3 minutes until browned and tender.
Toast the sourdough bread slices until crisp and golden.
Spread the Dijon mustard evenly on one slice of the toasted bread.
Layer the fresh arugula, tender beef strips, and caramelized onions on top of the mustard.
Place the second slice of bread on top and serve immediately while warm.