Baked Zucchini Sticks and Sweet Onion Dip

butter

medium sweet onion, about 1/2 pound, peeled and sliced

cider vinegar

honey

prepared mustard

mayonnaise

salt and pepper

medium zucchini, unpeeled, cut into 3"-long stick

salt

Panko bread crumbs

scant 1/2 cup freshly grated Parmesan cheese

Pizza Seasoning or mixed Italian herbs

olive oil spray

egg substitute; or 2 large eggs; or 3 egg whites, lightly beaten