In a medium skillet over medium heat, melt 1 tablespoon of the softened butter. Add the sliced onion and cook, stirring occasionally, until the onions are soft and caramelized, about 15-20 minutes.

Stir in the balsamic vinegar and brown sugar, cooking for an additional 5 minutes until the mixture thickens slightly. Season with salt and pepper to taste. Remove from heat and let cool slightly.

In a bowl, combine the cooled onion mixture with the remaining softened butter, mixing well. Form into a log, wrap in 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 to create a paste. Rub this mixture generously over both sides of the Porterhouse steaks.

Place the steaks on the grill or skillet and sear for about 4-5 minutes on each side for medium-rare, or until your desired doneness is reached. Remove the steaks from heat and let them rest for 5-10 minutes.

Slice the steaks and serve topped with a generous dollop of the balsamic-caramelized onion compound butter.