Add the onions, water, 3 tablespoons olive oil and 1 teaspoon salt to a large pan or pot and mix together. Turn the heat to medium-high and cook covered for 12 to 15 minutes, until the onions are soft and most of the water has evaporated.

Uncover the pan and turn the heat down to medium. Keep sautéing until the onions are deeply caramelized, about 30 more minutes. Watch the onions, adjust the heat or add a splash of water to prevent burning.

Mix the mayonnaise, sour cream, chipotle peppers and adobo sauce together and taste. Season with salt and pepper if needed.

Preheat the oven to 450°F with the rack in the middle. Heat a large cast iron or stainless steel pan over medium-high heat.

Cut the skirt steak into halves or thirds so it fits the pan. Pat it dry with paper towels and season both sides with 2 1/2 teaspoons salt and the pepper.

Add the neutral oil to the pan and, once it is smoking, add the steak. Sear 3 minutes per side, until well browned and the steak reaches at least 145°F on a food thermometer.

Move the steak to a cutting board and tent with foil for at least 5 minutes. Cut it into 4-inch pieces, then slice against the grain into 1/4-inch thick slices.

Cut the baguette in half lengthwise and separate the halves. Set the bottom half on a foil-lined baking sheet. Spread on a layer of chipotle mayo, then layer the steak, the onions and the provolone. Set the top half on the sheet and brush or spread it with 1 tablespoon olive oil so it toasts.

Bake until the cheese melts, 3 to 5 minutes. Watch it carefully.

Spread chipotle mayo on the top half, add the arugula if using, and close the sandwich. Slice into 4 large sandwiches.