Preheat your grill to high heat, about 450–500°F, creating a direct heat zone for searing. Pat the ribeyes dry, coat lightly with oil, and season generously on all sides with steak seasoning, pressing it into the meat.

Place the steaks over direct heat and sear, flipping every 30 seconds to develop an even crust without overcooking the interior. Grill until the internal temperature reaches 120°F for medium-rare, then remove from heat and rest for 10 minutes to allow the juices to redistribute.

While the steak rests, melt butter in a cast iron skillet over medium heat and add the sliced red onion with salt. Cook for 12–15 minutes, stirring occasionally, until deeply golden brown and soft with rich caramelized edges.

In a bowl, combine parsley, cilantro, roasted garlic, lemon juice, salt, red pepper flakes, olive oil, and red wine vinegar. Stir well and adjust with additional oil or vinegar if needed until the chimichurri is spoonable and balanced.

Slice the rested steak thinly against the grain for maximum tenderness. Lightly toast the ciabatta rolls on the grill for 1–2 minutes until warm and slightly crisp.

Build each sandwich by layering sliced steak, caramelized onions, provolone cheese, a drizzle of balsamic glaze, and a generous spoonful of chimichurri. Close the sandwich, slice in half, and serve immediately while warm and melty.