01 -
Put onions, water, oil and salt together in a big pan. Cover and cook on medium-high about 12-15 mins till soft. Then take off the lid, turn heat down to medium, and keep cooking around 30 mins till they turn dark brown. Add a splash of water if they start sticking.
02 -
Blend the mayo, sour cream, chipotles and adobo sauce in a bowl. Add salt and pepper till it tastes good to you.
03 -
Get your pan really hot on medium-high. Trim the steak to fit, dry it with paper towels, and rub in salt and pepper. Toss in hot oil and cook about 3 mins each side till nice and brown. Let it rest under foil for 5 mins, then cut against the grain into thin strips.
04 -
Warm oven to 450°F. Cut the baguette open and place on a foil-covered tray. Spread spicy mayo on bottom half, then stack with meat, brown onions, and cheese. Brush some oil on the top bread part. Bake for 3-5 mins till cheese gets melty. Throw on arugula if you want, close it up, and cut into pieces.