Thinly slice the yellow onion into uniform half-moons.
Heat the olive oil in a small non-stick skillet over medium-low heat; add the onions and a pinch of salt.
Cook the onions slowly, stirring occasionally, until they are soft and deep golden brown, which takes about 15 to 20 minutes.
In a mixing bowl, gently combine the ground beef with the sea salt, black pepper, and garlic powder, taking care not to overwork the meat.
Form the beef into one large, consistent patty about 1-inch thick.
Heat a cast-iron skillet or grill pan over medium-high heat until very hot.
Place the patty in the pan and sear for 4-5 minutes per side, or until the internal temperature reaches 160°F for a well-done finish.
While the beef rests for 2 minutes, lightly toast the whole wheat bun in the same skillet to soak up the savory flavors.
Spread the Dijon mustard on the bottom bun, then layer with the romaine lettuce and tomato slice.
Place the beef patty on top, crown with the warm caramelized onions, and finish with the top bun.