Preheat your oven to 400°F (200°C). Wrap a whole garlic bulb in aluminum foil and roast it for 40-60 minutes until soft and caramelized. Once cooled, squeeze the garlic cloves into a bowl and mash them with 4 tablespoons of unsalted butter until well combined.

Slice your crusty sourdough bread in half horizontally. Scoop out a small portion from the top half to create a well for your ingredients. Spread the garlic butter inside both halves.

In a skillet over medium heat, add 2 tablespoons of olive oil and thinly sliced yellow onions with a pinch of kosher salt. Sauté for about 25 minutes until caramelized.

Combine parsley, 1 tablespoon oregano, 2 minced garlic cloves, 2 tablespoons red wine vinegar, and a pinch of salt in a food processor. Blitz and drizzle in olive oil while mixing.

Bring ribeye steaks to room temperature for 30 minutes. Season with kosher salt and black pepper. Preheat grill to high and cook steaks for 3-4 minutes per side.

After cooking, rest steaks for 5 minutes. Heat a grill pan and place sourdough halves on the grill, buttered side down, for about 2-3 minutes until golden brown. Add mozzarella on the bottom half.

Spread chimichurri on the top half of the grilled sourdough. Place grilled ribeye and caramelized onions on the bottom half. Close the sandwich and slice in half.