In a medium bowl, combine mayonnaise, sugar, and chipotle in adobo sauce and lime juice. Mix until well combined and set aside.
Heat a large skillet over medium-high heat and add the turkey breast slices. Cook for about 4 minutes per side or until lightly browned on both sides.
Reduce the heat to low, add the sweet onion slices and cook for an additional 8 minutes or until caramelized (browned). Remove from heat and set aside.
Heat a grill pan over high heat and place the focaccia wedges onto it. Grill for 1 minute per side or until lightly charred lines begin to appear on each side of the focaccia. Remove from heat and set aside.
To assemble the sandwiches, you will need four focaccia wedges for each sandwich. Spread a generous amount of the mayonnaise mixture over each wedge and top with the cooked turkey breast slices, caramelized onions, and field greens. Place the remaining two focaccia wedges on top of these ingredients to form a sandwich.
Serve immediately or wrap up in parchment paper for later consumption.