Thinly slice the yellow onion into half-moons.
Heat the olive oil in a skillet over medium-low heat and add the onions, cooking slowly until they are soft and golden brown.
Stir the balsamic vinegar into the onions during the final minute of cooking to deglaze the pan, then set aside.
Season the flank steak evenly on both sides with sea salt and black pepper.
Increase the skillet heat to medium-high and sear the steak for 3-4 minutes per side for medium-rare.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain.
In a small bowl, whisk together the plain Greek yogurt and horseradish to create a creamy spread.
Toast the sprouted grain bread slices until they are lightly crisp and golden.
Spread the horseradish sauce on one slice of bread, then layer with the arugula, sliced steak, and caramelized onions.
Top with the remaining slice of bread and cut diagonally to serve.