Grill the chicken:
Heat a gas grill to medium high or prepare a medium-hot charcoal fire. Oil the grill grate. Spread the spice rub on a plate. Lightly rub each chicken breast with oil and press each breast into the spice rub to thoroughly coat one side of the meat.
When the grill is hot, arrange the chicken, spice-rub side down, at a 5-degree angle to the bars of the cooking grate. Grill until the chicken has grill marks, about 2 minutes. Rotate the breasts 90 degrees and continue grilling for 2 minutes. Flip the chicken and cook the second side the same way for another 4 to 6 minutes, until the chicken has crosshatched grill marks and the meat is cooked through. Transfer the breasts to a platter, let cool until barely warm, cover loosely, and refrigerate until you're ready to pack your cooler or assemble the sandwiches.
Build the sandwiches:
Right before you leave for the picnic, or when you reach your destination, slice each breast thinly crosswise, holding the knife at a 45-degree angle. Cover the bottom pieces of focaccia with a layer of arugula (to keep the bread from getting soggy). Top with about a tablespoon of onion marmalade, some sliced chicken breast, a sprinkle of salt, more onion marmalade, and another layer of arugula. Cover with the top pieces of toasted focaccia. Wrap each sandwich tightly in plastic wrap for transport or serve immediately.
nutrition information (per serving):
Photo: Scott Phillips