Soak lentils overnight in cold water.
Cook in water in which they were soaked, adding seasoning, until tender.
Beat until smooth.
Heat fat in a frying pan, then fry sliced onion, chopped tomato and chopped apple.
When soft add this mixture to lentils, together with sage, oatmeal or breadcrumbs, extra seasoning if necessary and egg.
Put into greased tin and cover with greased paper.
Bake in centre of a moderately hot oven (400°F - Gas Mark 5) for about 45 minutes.
Serve with tomato or mushroom sauce.