Unroll the herrings, reserving the pickled onions from the marinade.
Cut the herring fillets into bite-sized pieces.
Scatter the rocket onto two individual plates or one large serving platter.
Arrange the herring pieces evenly over the rocket.
Peel and stone the avocado, then slice or dice the flesh. Arrange the avocado around the herring.
Halve the tomatoes and arrange them around the avocado pieces.
Drizzle with fresh lemon juice and a little olive oil, or use some of the vinaigrette from the herring marinade for extra flavour.
Scatter the reserved onion pieces on top, and serve immediately with sourdough bread.