Place potatoes in large saucepan and cover with cold water; bring to a boil. Cook 15 min. or until just tender. Drain and toss potatoes with olive oil, garlic, thyme, rosemary and 1/2 tsp (2 mL) salt. Wrap mixture in double-thick foil packet. Seal tightly.
Meanwhile, mix fennel seeds, mustard seeds, peppercorns and remaining salt. Rub all over steak. Let stand 10 min.
Place potato packet on grill preheated to medium-high. Cook 15 min., turning over packet halfway through cooking, until potatoes are tender. Meanwhile, grill steak 5 to 7 min. per side or until meat reaches 145°F (63°C) on an instant-read thermometer for medium-rare, or to desired doneness. Rest steak 10 min. before slicing and serving with potatoes.
To coarsely crush spices use a mortar and pestle, or crush under small, heavy-bottom skillet or saucepan.