Boil the potatoes until just tender. While this occurs, preheat an indoor grill or barbecue to medium-high. Combine the remaining salad ingredients in a medium bowl. When the potatoes are tender, drain well, place in the bowl and toss to combine. Tightly cover salad with foil and set aside.
Brush each halibut steak with the 2 Tbsp. olive oil; season with salt and pepper. Oil the bars of the grill. Grill the halibut 3 to 4 minutes per side, or until just cooked through. Set fish on serving plates. Place some of the warm potato salad alongside the fish, garnish with dill sprigs and lemon slices, and serve.