Heat the olive oil in a small skillet over medium-low heat.
Add the sliced yellow onions and cook slowly, stirring occasionally, until they are soft and golden brown, about 15 minutes.
Stir in the balsamic vinegar to the onions and cook for an additional 2 minutes, then remove from heat and set aside.
In a medium bowl, combine the ground beef with smoked paprika, garlic powder, sea salt, and black pepper.
Form the beef mixture into one thick patty or two smaller ones.
Preheat a grill or cast-iron skillet to medium-high heat and cook the patty for 4-5 minutes per side until the internal temperature reaches your desired doneness.
Place the butter lettuce leaves on a plate to act as the base bun.
Layer the grilled beef patty onto the lettuce, then top with Dijon mustard, the tomato slice, and the caramelized onions.
Wrap the lettuce leaves around the burger and serve immediately while warm.