Thinly slice the yellow onion into half-moons.
Heat the extra virgin olive oil in a skillet over medium-low heat and add the onions with a pinch of sea salt.
Sauté the onions for 12-15 minutes, stirring occasionally, until they are soft and golden brown.
Stir in the balsamic vinegar to deglaze the pan, cook for 1 more minute, then remove the onions from the skillet and set aside.
While onions cook, form the ground brisket into a single thick patty and season both sides with the remaining sea salt and black pepper.
Increase the skillet heat to medium-high and sear the brisket patty for 4-5 minutes per side, or until it reaches your desired level of doneness.
During the last minute of cooking, place the sprouted grain bun halves cut-side down in the pan to lightly toast.
Assemble the burger by spreading Dijon mustard on the bottom bun, followed by the baby arugula, the brisket patty, and the caramelized onions.