In a food processor, combine dried porcini mushrooms and toasted walnuts. Pulse until finely ground but not a powder. Transfer the mixture to a bowl and mix in salt, pepper, thyme, and garlic powder. Set aside.

In a saucepan over medium heat, add sliced sweet onions and balsamic vinegar. Cook slowly, stirring occasionally until the onions are caramelized and the vinegar has reduced, about 15-20 minutes. Remove from heat and let cool slightly.

In a mixing bowl, combine the softened butter and the onion-balsamic mixture. Season with salt and pepper. Mix well and transfer to a piece of parchment paper. Roll into a log shape and refrigerate until firm.

Preheat your oven to 400°F (200°C). Pat the ribeye steaks dry with paper towels. Season both sides generously with salt and pepper.

Heat olive oil in a large, oven-safe skillet over medium-high heat. Once hot, add the ribeye steaks and sear for about 3-4 minutes on each side until a golden brown crust forms.

Remove the skillet from heat. Press the porcini and walnut mixture onto the top of each ribeye steak, ensuring an even coating.

Transfer the skillet to the preheated oven and roast for about 6-8 minutes for medium-rare, or until desired doneness is reached.

Remove the skillet from the oven and let the steaks rest for about 5 minutes. Slice the sweet onion and balsamic confit butter and place a piece on top of each steak just before serving. Garnish with chopped fresh chives and serve alongside your favorite sides.