OK - wrong book! It's from Rose Elliot's "Not Just a Load of Old Lentils"
Lentils and Mushrooms au Gratin
6 oz lentils
1 pint milk and water mixed
2 oz butter
1 large onion
juice and rind of 1/2 lemon
salt and pepper
1 teaspoon yeast extract (Marmite)
8 oz mushrooms
1-2 oz each of fresh breadcrumbs and grated cheese for topping
Soak the lentils in the liquid overnight or for a few hours if possible (I don't do this - I make sure that the pint of water/milk is really hot and then pour it over the lentils and leave to soak for about 30 minutes)
Simmer in the liquid until golden and tender
Meanwhile melt half the butter, add sliced onion and cook until tender but not brown
Add to lentils with lemon juice and grated rind, salt pepper and yeast extract; liquidise to make smooth, thick puree
Slice the mushrooms and fry in the rest of the butter until just tender
Place the mushrooms in a shallow casserole, top with the lentil mixture and the breadcrumbs and cheese
bake in a moderate oven, 350/GM4 for 40-45 minutes until golden and bubbly
Apologies that it's imperial rather than metric, but it's an old book!
I seem to remember that I used a lot more mushrooms the last time I made it, but I guess it's up to you.