Break apart halibut meat with two forks until no large chunks remain. Add beaten egg and mix well with your hands. (Your hands will be quite involved for this recipe.)
Add onions, herbs, salt, pepper, and ½ cup breadcrumbs, mixing again with your hands.
Spread remaining breadcrumbs evenly on a plate and have another plate ready. Take a half-cup measure and fill it to heaping with the halibut mixture. Form mixture into a large patty with your hands, pressing so that it keeps its shape. Roll the patty in crumbs and place on the empty plate. Repeat with the remaining mixture until you have 4 large patties. Refrigerate patties for 1–2 hours before cooking.
Heat butter and oil in a 10-inch frying pan over medium heat. When butter is sizzling, add halibut patties. Cook on each side for about 4 minutes, until burgers are firm and golden.
Serve on homemade sourdough buns with juniper tartar sauce and all the trimmings.
