In saucepan, melt butter over medium heat. Add onion; saute 2 minutes. Stir in syrup, vinegar. Boil until reduced to 1 cup, 8 minutes. Scrape into bowl; whisk in mustard and salt. Chill 30 minutes to thicken.
In cup, mix sage, salt and pepper. Rub over tenderloins. Cut squash crosswise into 1/2-inch-thick rings. Discard seeds, leaving rings in one piece.
Prepare outdoor grill with hot coals, or heat oven broiler, or heat grill pan. (If glaze is too thick, microwave at 100 percent power to loosen, 30 seconds.)
Grill pork and squash 5 minutes; turn over; brush with glaze. Continue to grill, turning and brushing every 5 minutes for the first 15 minutes of grilling, until pork registers 160 degrees F and squash is tender, 20 minutes total. Discard any remaining glaze.