Make the marinade by coarsely grinding the juniper berries and peppercorns in a spice grinder or using a pestle and mortar. Mix with the sugar and oil.
Use tweezers to remove any small bones from the salmon fillets. Brush the fillets with the marinade and leave to marinate at room temperature for 1 hour.
Wrap each salmon fillet in a piece of foil, sealing the edges well. Cook on a hot barbecue for 8—10 minutes, then set aside to rest briefly. Unwrap the salmon, garnish with lemon wedges, parsley sprigs and spring onions and serve with frisée lettuce leaves.