Sprinkle with dry soup mix. Next, stuff the core of the onion with a beef bullion cube and butter then wrap in foil and grill for about 40 minutes. with your knife, slice onion from top almost to the bottom in 1/8s, again, leaving onion in tact. "Blossom" the onion and place pads of butter between folds. Just peel and core your onion like an apple. Peel back and trim outer layer of Vidalia skin, cutting off the top and the bottom of onion so that it can sit upright. Peel onion and cut off the stem from the bottom, leaving the onion intact. Place foil packets on grill; cook 20 to 30 minutes (if grill becomes too hot, move onions to a cooler area), until tender and slightly charred. Heat grill to medium hot. Place one teaspoon of sugar on top of the onion (this should look like a little mound of sugar). The process to make foil-wrapped grilled onions is really easy. Place onion on a sheet of aluminum foil and season generously with kosher salt. Let onions cool in packets for 15 minutes.