In a medium skillet over medium heat, arrange the bacon slices in a single layer. Cook for about 10 minutes, flipping occasionally, until the bacon is crispy and golden brown.

Carefully leave about 2 tablespoons of bacon grease in the skillet for flavor. Add the sliced onions to the pan and cook for approximately 10 minutes, stirring occasionally, until golden brown.

In a small mixing bowl, combine mayonnaise with adobo sauce and mix until smooth. Adjust spice level to taste.

Toast the sandwich bread in the same skillet for 1-2 minutes per side or until lightly golden.

Spread chipotle mayo on one slice of toasted bread, layer with bacon, caramelized onions, tomato slices, and arugula.

Top with another slice of toasted bread, press gently, slice in half, and serve immediately.