In a large cast-iron skillet, melt the butter over medium-low heat. Add the sliced onions, water, and salt. Cook for 10-15 minutes, stirring occasionally until the onions turn golden brown and fragrant. Add minced garlic and cook for an additional minute. Set aside and keep warm.

Season your ribeye steak with coarse-ground salt and pepper on both sides. In the same skillet over medium-high heat, cook the steak for about 3-4 minutes on each side until the internal temperature reaches 130°F for medium rare. Let it rest for 5 minutes before slicing thinly against the grain.

Preheat your oven's broiler. Cut the baguette in half lengthwise and place the bottom half on a baking sheet. Layer the sliced ribeye steak evenly on the bread and top with provolone cheese. Broil for about 2-3 minutes, or until the cheese is melted and bubbly.

Remove the baguette from the oven. Layer the caramelized onions over the steak. In a small bowl, toss fresh arugula with olive oil and lemon juice. Add the arugula on top of the onions before placing the top half of the baguette over everything.

Spread a layer of garlic aioli on the cut side of the top half of the baguette. Carefully place it atop the sandwich and press down gently. Cut into portions and serve immediately.