Cut off top one-third of bread loaf, and spread cut side of top with honey mustard; set aside.
Scoop out soft center of remaining bread, leaving a 1/4-inch-thick shell. (Reserve soft center of loaf for other uses, if desired.)
Stir together onions, cream cheese, and mayonnaise; spread in bottom of bread shell. Layer with ham, peppers, and cheeses; cover with bread top, honey mustard side down. Wrap in aluminum foil or a heavy-duty zip-top plastic bag. Place tomato and lettuce in zip-top plastic bags. Surround with ice packs for transport to picnic.
Unwrap whole sandwich, add lettuce and tomato slices, and cut into 4 wedges. Serve on heavy paper towels or parchment paper.