Squeeze the juice from half a lemon into a bowl and set aside.
In a deep pan place the halved lemons, onion quarters and enough water to fill the skillet 2-3 inches deep. Bring the water to a boil.
Reduce the temperature to low so that the water is at a gentle simmer. Season the salmon with salt and lower it into the pan. Cover with a lid and let cook for approximately 5 minutes until the fish is opaque in colour. Remove the salmon from the water and allow to cool.
In a bowl whisk together the mayonnaise, capers, green onions, red onions, dill and lemon juice. Use a fork to break the salmon fillets into small flakes. Add the flaked salmon to the mayonnaise mixture and stir until well combined. Taste and add salt if necessary.
Store the salmon spread in the fridge for at least 30 minutes to allow the flavours to blend. It can be made up to a day in advance.
Serve the spread with crackers or on cucumber slices topped with a sprig of fresh dill.