Thinly slice the yellow onion into half-moons.
Heat half of the olive oil in a large skillet over medium-low heat.
Add the onions to the skillet and cook slowly, stirring occasionally, until they become soft and golden brown.
Pour in the balsamic vinegar to deglaze the pan, scraping up any browned bits, then remove onions and set aside.
In a medium bowl, combine the ground beef with sea salt, black pepper, and garlic powder.
Shape the seasoned beef into two thick, even patties.
Wipe the skillet clean and heat the remaining olive oil over medium-high heat.
Place the beef patties in the skillet and sear for approximately 4 to 5 minutes per side until cooked through.
Remove the patties from the pan and immediately add the fresh baby spinach to the hot skillet, sautéing for 1 minute until just wilted.
Serve the beef patties topped with the warm caramelized onions with the sautéed spinach on the side.