Line the slow cooker with baking paper. Place the fish on the paper and scatter with onion.

Combine the pesto with ¼ cup water and pour the mixture over the fish. Scatter with tomato.

Cover, using the tea towel trick, and cook on high for 2 hours or until the fish is cooked through.

Transfer carefully to serving plates using an egg flip – the fish will be very fragile.

Spoon some of the pesto sauce over the fish and garnish with the tomato.

Serve with a fresh salad and fries/wedges