Brush onion slices with some of the dressing; set aside. Cut tops off peppers and discard. Remove seeds and membranes from peppers. Cut peppers crosswise into 1/4-inch thick slices. Toss pepper slices with remaining salad dressing in a large bowl.

For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat over drip pan. Prick sausage links with a fork in several places. Place sausages on grill rack over drip pan and onion slices directly over the coals around edge of grill rack. Cover and grill 10 minutes. Turn sausage and onion slices. Place pepper slices on grill rack directly over the coals. Cover and grill 5 to 15 minutes more, turning peppers once and removing vegetables from the grill when they are crisp-tender. Cook sausages until the internal temperature registers 160 degree F when tested with an instant read thermometer. (For a gas grill, preheat grill. Reduce heat to medium high. Add vegetables and sausage to grill as above. Cover and grill as above.) Remove sausages from grill.

Meanwhile, combine mayonnaise and mustard; spread on cut sides of focaccia. Bias-slice sausage 1/4-inch thick. Separate onion slices into rings, if necessary. Arrange sausage, peppers, and onions on focaccia bottom. Add focaccia top. Press down gently on focaccia top to slightly compress sandwich. Cut into wedges to serve. Makes 6 servings.

Place vegetables with dressing in a covered container. Place mayonnaise mixture in a separate covered container. Tote containers with vegetables and mayonnaise mixture and sausage links in an insulated cooler with ice packs. Grill as above at tailgating site.

For easy serving, place sandwiches in waxed sandwich bags.