In a skillet over medium heat, melt the butter. Add the sliced onions and cook, stirring occasionally, until they are caramelized and golden brown, about 15-20 minutes.

Stir in the balsamic vinegar and cook for an additional 2-3 minutes until slightly thickened. Remove from heat and let cool slightly.

In a bowl, combine the caramelized onions with the remaining butter, mixing well. Season with salt and pepper to taste. Form the butter into a log shape using parchment paper and refrigerate until firm.

Preheat your grill or a cast-iron skillet over medium-high heat.

In a small bowl, mix the minced garlic, chopped sage, olive oil, salt, and pepper. Rub the garlic-sage mixture generously over both sides of the ribeye steaks.

Place the ribeye steaks on the preheated grill or skillet. Cook for about 4-5 minutes on each side for medium-rare, adjusting the time according to your desired doneness.

Use a meat thermometer to check for an internal temperature of 130°F (54°C) for medium-rare.

Once cooked to your liking, remove the steaks from the heat and let them rest for about 5 minutes.

Slice the compound butter into rounds and place on top of the warm ribeye steaks. Garnish with chopped parsley and serve immediately.