Skin the salmon and remove any stray bones. Cut it into chunks, roughly 1in/2.5cm square and put in a bowl, container or freezer bag. Finely chop or grate the garlic and ginger.
Mix together the soy sauce, mirin, sake, brown sugar, garlic and ginger so they are well blended together then pour in with the fish. Leave to marinade for at least 10 minutes, but ideally 30 minutes, turning once or twice to ensure all the fish takes on the marinade if the pieces are not well coated to start.
Cut the white and pale green part of the green onions in to lengths around 1 inch (2.5cm) long or slightly longer (if using).
Thread pieces of onion and salmon on to two longer or four shorter skewers, or more/less depending on size, roughly one piece of onion followed by one to two pieces of salmon. Put a piece of onion on both ends to hold everything on.
Lightly oil (with brush or spray) your grill/grill pan. Grill the skewers for around 5 - 10 minutes over medium heat, depending on your grill/barbecue and size of the fish chunks. Keep a close eye as they cook - you should turn once the fish is cooked through on the bottom side and you start to see the color chance around the sides. Remember, the second side will cook more quickly. If you can't or prefer not to grill/bbq, you can also cook on a grill pan or put under broiler/grill. Take care not to break up the fish as you turn the skewers.