Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Slice the sweet potato into uniform wedges and toss them in a bowl with the olive oil and a pinch of sea salt.
Spread the wedges in a single layer on the prepared baking sheet and roast for 20-25 minutes, flipping halfway through, until tender and lightly browned.
While the potatoes roast, place the ground beef in a bowl and season with garlic powder, sea salt, and black pepper.
Gently form the beef into a thick, uniform patty, being careful not to overwork the meat.
Heat a cast-iron skillet or non-stick pan over medium-high heat and sear the patty for 4-5 minutes per side until it reaches your desired level of doneness.
Wash and pat dry the butter lettuce leaves to use as your bun substitute.
Place the cooked patty onto the lettuce leaves and top with the tomato slice, red onion, and yellow mustard.
Serve the burger immediately alongside the hot, roasted sweet potato wedges.