With a sharp knife, remove skin and white pith from oranges and lemons. Working over a bowl, separate oranges and lemons into segments and set aside. Reserve juice.
Thinly slice spring onions. Set aside a few light green rings for garnish.
Cut herring fillets into small pieces and arrange on a platter with citrus segments and spring onions.
To make citrus vinaigrette, whisk oil, reserved juice, orange juice, lemon juice and vinegar until combined; add salt and pepper to taste. Drizzle over salad. Cover and refrigerate at least 2 hours to allow flavours to develop.
Remove salad from refrigerator and stand until at room temperature. Garnish with spring onion rings and lemon balm. Serve with pumpernickel or dark wholegrain bread.
Mostly sold vacuum packed, frozen, or in cans, matjes herring is a richly flavoured oily fish that's full of omega-3 fatty acids. Ready-to-eat matjes herring have been skinned, gutted and filleted.