In a small bowl, mix together mayonnaise, minced chipotle peppers, adobo sauce, minced garlic, lime juice, and salt. Adjust seasoning according to your taste. Set the sauce aside in the refrigerator until you're ready to use it.
Heat olive oil over medium-low heat in a large cast-iron skillet. Add the thinly sliced red onions to the skillet and sauté them for about 5 minutes until they begin to soften. Reduce the heat to medium-low. Season with salt and continue cooking the onions, stirring occasionally, until they turn deeply golden brown and caramelize. Set them aside once they are caramelized.
Heat olive oil in a skillet over medium-high heat. Once hot, add the steak strips in a single layer, making sure not to overcrowd the pan. Season the ribeye strips with salt and black pepper and cook the steak for about 2-3 minutes per side, or until they are cooked to your desired level of doneness.For medium-rare, aim for an internal temperature of 130°F (54°C). Remove the strip steaks from the heat and let them rest. Preheat your oven to 350 degrees F (175 degrees C) and place the sliced baguette on a baking sheet and toast in the oven for 5-7 minutes until it turns lightly brown and crispy (optional).
Spread a generous amount of chipotle mayo on one of the toasted baguette halves.
Arrange the cooked slices of steak on top of the mayo.
Layer sliced cheese on top of the steak strips.
Top the steak with caramelized onions and any additional toppings such as banana peppers, mustard, or peppery arugula.
Spread a generous amount of chipotle mayo on the other toasted baguette half and close the sandwich, pressing it down gently.
Slice the juicy steak sandwich in half if necessary and serve immediately.