01 -
Put your sliced onions in a big pan with water, oil and salt. Cover and cook on medium-high about 12-15 mins till they're soft. Then take the lid off, turn heat down to medium, and keep cooking around 30 mins until they turn dark brown. Add splashes of water whenever they start to stick.
02 -
Throw mayo, sour cream, chipotles and adobo sauce in a bowl and mix it all together. Add salt and pepper till it tastes good to you.
03 -
Get your pan super hot on medium-high. Cut the steak to fit your pan, dry it with paper towels, and sprinkle salt and pepper all over. Toss it in hot oil and cook about 3 mins each side till it's nice and brown. Let it rest under foil for 5 mins, then cut across the grain into thin slices.
04 -
Turn oven to 450°F. Cut the baguette open and lay it on a foil-covered baking sheet. Spread the bottom with spicy mayo, stack on steak, top with brown onions, and cover with cheese. Brush some oil on the top bread half. Bake for 3-5 mins till the cheese gets all melty. Toss on arugula if you want, close it up, and cut into pieces.