Heat half the butter in a heavy, non-stick frying pan over a medium heat.  Add the onion and cook for few minutes, until soft and transparent.  Remove to a plate with a slotted spoon.

Add the rest of the butter to the pan and when melted sear the liver for 1-2 minutes, turn and cook for another 1-2 minutes on the other side.

Return the onion to the pan, squeeze in the lemon juice and heat through.  Add the chopped sage and season to taste with salt and pepper.

Serve with green leaf salad and crusty bread.