- First, make a pouch for the potatoes: Fold a 3-foot length of heavy-duty aluminum foil in half. Fold over the edge of each long side three times, making each fold 1/2 inch wide. Repeat the process to seal the folded short end.
- In a large bowl, whisk together the mustard, the 3 tablespoons grilling drizzle, the lemon juice, stock, salt and pepper. Add the potatoes and stir to combine. Spread the butter evenly inside the pouch and place the potatoes inside. Seal the pouch by folding over the edge three times, as instructed above.
- Preheat an electric grill to medium-high heat according to the manufacturer's instructions. Place the potato pouch on the grill and cook, carefully turning the pouch over every 5 minutes, until the potatoes are tender and cooked through, about 20 minutes. Transfer the sealed pouch to a warmed platter.
- Brush the halibut with the grilling paste and season with salt and pepper. Drizzle with a little olive oil. Arrange the halibut on the grill and cook, turning once, until dark grill marks appear underneath and the fish is opaque throughout, about 3 minutes per side.
- Remove the potato salad from the pouch and arrange on the platter. Place the halibut on top. Sprinkle additional grilling drizzle over the fish and serve immediately.