Heat the extra virgin olive oil in a small skillet over medium-low heat.
Add the thinly sliced yellow onions to the skillet and cook, stirring occasionally, until they are soft and deep golden brown, approximately 15 to 20 minutes.
While the onions caramelize, divide the ground beef and form it into one large, thick patty, being careful not to overwork the meat.
Season both sides of the beef patty generously with sea salt and black pepper.
Heat a cast-iron skillet or grill pan over medium-high heat until very hot.
Place the patty in the hot skillet and sear 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 butter lettuce, tomato slice, beef patty, and a heap of caramelized onions.