Flake the mackerel into a bowl, discarding any skin and pin bones you can spot.
Add the softened butter and the lemon juice and mash everything together with a fork until you have a rough, spreadable pâté. A few chunks of fish left in it is a good thing.
Taste and add a generous grind of black pepper. It is unlikely to need salt because the fish is salty already.
Chill the pâté in the fridge for 15 minutes if you have time, which firms it up and lets the flavours come together.
Toast the bread and spread the pâté thickly while the toast is still hot. Finish with another grind of pepper and serve straight away.