Grilled Beef Tenderloin with Potato Hobo Packs

2 tablespoons black peppercorns 1 1/2 tablespoons kosher salt 2 tablespoons olive oil 1 (2 1/2-lb) center-cut beef tenderloin roast, trimmed and tied 1 1/2 lb fingerling or baby Yukon Gold potatoes (about 16), halved lengthwise 1/2 lb pearl onions (about 16), blanched in boiling water 1 minute, drained, and peeled 12 garlic cloves 2 tablespoons minced fresh sage 1/4 cup extra-virgin olive oil 1/2 cup sour cream 6 bacon slices, cooked and crumbled 1/4 cup thinly sliced fresh chives Special equipment: 12-inch-wide heavy-duty aluminum foil; a 22 1/2-inch kettle grill with a hinged top rack (if using charcoal); a large chimney starter (if using charcoal); long metal tongs; a 17- by 12 1/2- by 3 1/2-inch disposable aluminum roasting pan (if using charcoal); an instant-read thermometer