Peel the yellow onion and slice it into very thin half-moons.
Heat the olive oil in a non-stick or cast-iron skillet over medium-low heat.
Add the sliced onions to the skillet along with a pinch of the sea salt to help draw out moisture.
Cook the onions slowly for 15 to 20 minutes, stirring occasionally, until they are soft and have turned a deep golden brown.
Stir in the balsamic vinegar to deglaze the pan, scraping up any browned bits, and cook for 1 more minute until the liquid has evaporated.
Remove the onions from the skillet and set them aside on a plate.
Increase the skillet heat to medium-high and place the Beyond Burger patties in the pan.
Sear the patties for 3 to 4 minutes on the first side until a nice crust forms, then flip.
Season the cooked side with the remaining sea salt and black pepper.
Cook for another 3 to 4 minutes on the second side until the internal temperature reaches 165°F.
Lay the butter lettuce leaves flat on a serving plate to act as your 'buns'.
Place one patty on each lettuce leaf and divide the caramelized balsamic onions evenly over the top of both patties.