Combine the sliced onions, water, olive oil, and salt in a large pan or pot, cover, and cook over medium-high heat for 12-15 minutes. The onions should turn soft while most of the water cooks away.

Remove the lid, lower the heat to medium, and cook the onions for about 30 minutes, stirring as they brown. They are ready when glossy, deeply amber, and soft enough to mound like onion jam; add a splash of water if the pan threatens to scorch.

Stir the mayonnaise, sour cream, chipotle peppers, and adobo sauce together until the mixture is smooth. Taste the spread and season with salt and pepper as needed; it should taste creamy first, then smoky and warmly spicy.

Set an oven rack in the middle position and heat the oven to 450°F, then warm a large cast iron or stainless steel skillet over medium-high heat. The skillet should be fully hot before the oil goes in, which helps the steak brown instead of releasing liquid.

Cut the skirt steak into halves or thirds as needed to fit the skillet, pat it dry, and season both sides with the salt and black pepper. A dry, evenly seasoned surface is the starting point for a dark, savory crust.

Add the neutral oil to the hot skillet and lay in the steak once the oil begins to smoke, then sear for 3 minutes per side. Expect a loud sizzle and a deep mahogany exterior; move the steak to a cutting board, tent loosely with foil, and rest for at least 5 minutes.

Cut the rested steak into 4-inch pieces, then carve each piece into 1/4-inch slices across the grain. The slices should show shortened fibers and bend easily rather than pulling into long, chewy strands.

Split the baguette lengthwise and place both halves on a foil-lined baking sheet, with the bottom half facing up. Keep the bread halves separated so the cheese can melt while the top develops a crisp olive-oil sheen.

Spread chipotle mayo over the bottom half, then arrange the sliced steak, caramelized onions, and provolone on top; brush the cut side of the top half lightly with olive oil. Layering the warm ingredients over the sauce keeps the sandwich juicy while giving the bread a better chance of staying firm.

Bake for 3-5 minutes, watching closely, until the provolone has melted. The cheese should look bubbly and drape over the steak, while the bread edges turn toasted rather than dark brown.

Spread chipotle mayo over the toasted top half, add the baby arugula, close the sandwich, and cut into 4 portions. The arugula should stay bright and lightly peppery against the warm beef and sweet onions.