Coarsely crack peppercorns (see cooks' note, below). Stir together pepper and salt in a small bowl, then add oil and stir until combined well. Rub spice mixture all over beef.
Tear off 8 (2-foot-long) sheets of heavy-duty foil and arrange in 2 stacks of 4. Divide potatoes, onions, garlic, and sage between stacks, arranging in 1 layer in center of foil, then drizzle each with 2 tablespoons oil and season with salt and pepper. Working with top sheet of foil, fold each side over potato mixture to enclose, then turn package a quarter turn and repeat folding and turning package with each piece of remaining foil. Make second hobo pack in same manner.
To cook beef and hobo packs using a charcoal grill: Open vents on bottom of grill. Light charcoal (80 to 100 briquettes) in chimney starter. Leaving about one quarter of grill free of charcoal, bank lit charcoal across rest of grill so that coals are about three times higher on opposite side.
Charcoal fire is medium-hot when you can hold your hand 5 inches above rack over area where coals are piled highest for 3 to 4 seconds. Using long metal tongs, lift up one hinged side of rack and put hobo packs, folded sides down, on portion of bottom rack with no coals on it. (One side of packages will be in direct contact with coals.) Cook hobo packs, turning a quarter turn with tongs every 15 minutes, 45 minutes total.
While hobo packs are cooking, sear beef on lightly oiled grill rack directly over hottest part of coals, uncovered, turning occasionally and, if necessary, moving around grill to avoid flare-ups, until well browned, 12 to 15 minutes total. Move beef to coolest part of grill, then cover with inverted roasting pan and grill, turning occasionally, until thermometer inserted diagonally into center registers 120°F for medium-rare, about 10 minutes. Transfer beef to a cutting board and let stand, loosely covered with foil, 15 minutes.
To cook beef and hobo packs using a gas grill: Preheat all burners on high, covered, 10 minutes. Reduce heat to moderately low on 2 burners if your grill has 3 (or 1 burner if your grill has 2) and cook hobo packs, folded sides down, on grill rack, covered with lid, over moderately low burner(s) 40 minutes.
Meanwhile, sear beef on lightly oiled grill rack over remaining burner on high, covered with lid, turning over occasionally, until well browned, 12 to 15 minutes total. Reduce heat to moderate and cook beef, covered with lid, turning over occasionally, until thermometer inserted diagonally into center registers 120°F for medium-rare, about 10 minutes. Transfer beef to a cutting board and let stand, loosely covered with foil, 15 minutes.
Serve beef and hobo packs: Thinly slice beef, then remove potato mixture from packages and serve with sour cream, bacon, and chives.
Cooks' notes: To crack peppercorns, coarsely crush with a mortar and pestle or wrap in a kitchen towel and press with bottom of a heavy skillet. If you aren't able to grill outdoors, beef can be seared in a hot lightly oiled well-seasoned ridged grill pan over moderately high heat, turning occasionally, until well browned, about 15 minutes total, then transferred to a large roasting pan with potato mixture scattered around it and roasted in middle of a preheated 450°F oven until thermometer registers 120°F, 20 to 25 minutes. Transfer beef to a cutting board and continue to roast potato mixture until vegetables are tender, about 5 minutes more. Keep vegetables warm in a bowl, covered, while beef stands.