Thinly slice the yellow onion into half-moons.
Heat 0.5 tsp of olive oil in a skillet over medium-low heat and add the onions.
Cook the onions for 12-15 minutes, stirring occasionally, until soft and golden brown.
Stir in the balsamic vinegar during the last minute of onion cooking, then remove from the pan.
Season the steak on both sides with sea salt and black pepper.
Increase the skillet heat to medium-high and add the remaining 0.5 tsp of olive oil.
Sear the steak for 3-4 minutes per side for medium-rare, then let it rest on a cutting board for 5 minutes.
Slice the baguette roll in half and lightly toast it until the edges are crisp.
Thinly slice the rested steak against the grain to ensure maximum tenderness.
Spread the Dijon mustard on the bottom half of the roll, then layer with sliced steak, caramelized onions, and fresh arugula before closing the sandwich.