In a medium-sized skillet, heat over medium heat and add your thick-cut bacon strips. Cook for about 8–10 minutes, flipping occasionally until the bacon is crispy and deeply golden. Once done, transfer the crispy bacon to a paper towel to drain excess grease, while reserving about 2 tablespoons of the bacon fat in the pan for the next steps.

Using the reserved bacon fat in the skillet, add the thinly sliced onions. Cook them over medium-low heat for 10-15 minutes, stirring occasionally until they become soft and caramelized to a rich golden brown.

In a small bowl, combine mayonnaise and the smoky adobo sauce, mixing well until smooth. Taste and adjust the heat level by adding more adobo sauce if desired.

Wipe the skillet clean and return it to the stovetop. Over medium heat, add the remaining bacon fat and place your slices of hearty bread in the pan. Toast each side for 1–2 minutes until golden brown.

On one slice of the toasted bread, spread a generous layer of the chipotle mayo. Next, layer on the sweet caramelized onions followed by the crispy bacon. Top with freshly sliced tomato and a handful of peppery arugula. Close with the second slice of bread and serve immediately.