In a small bowl, combine the ground beef with sea salt, black pepper, and garlic powder, then form into two equal-sized patties.
Heat the olive oil in a large skillet over medium-low heat and add the thinly sliced yellow onions.
Cook the onions slowly for about 10-12 minutes, stirring occasionally, until they are soft and deep golden brown.
Stir in the balsamic vinegar to deglaze the pan, scraping up any browned bits, then remove the onions from the skillet and set aside.
Increase the skillet heat to medium-high and add the ghee to the pan.
Place the beef patties in the hot skillet and sear for 4-5 minutes per side, or until they reach your desired level of doneness.
During the last minute of cooking, add the fresh baby spinach to the empty spaces in the pan and toss briefly until just wilted.
Plate the patties topped with a generous portion of caramelized onions and serve the wilted spinach on the side.