This one started life as a vegetarian dish I found in that wonderful little book called Not Just a Load of Old Lentils by Rose Elliot.
Put a cup of orange split lentils (thoroughly rinsed) in a large saucepan. Add a cup of water and a cup of milk. Don't add any salt. Simmer for twenty minutes or so until the lentils are completely soft.
Meanwhile slice a large onion, and cook gently for ten minutes in another pan with a little olive oil. The onions should be very soft but not browned.
In a third pan, cook ten or so large mushrooms, cut into thick slices in a little more olive oil for ten minutes or so. Add plenty of freshly-ground black pepper. The mushrooms should be soft right the way through.
Add the onions to the cooked lentils, then add the juice of a lemon, a teaspoon of Marmite (or, if you prefer, a couple of dashes of Worcester sauce - not strictly vegetarian, though) and lots of freshly-ground black pepper. I find that salt is not needed here, since the Marmite tend to be quite salty on its own. Stir well.
Spread the cooked mushrooms over the bottom of a wide shallow oven-proof dish (ceramic or pyrex). Cover with the lentil mixture, then sprinkle a little grated cheese over the top. A sprinkle of paprika will add colour, if you want.
Cook in a pre-heated oven at 180C (Gas Mark 5) for at least 40 minutes. The cheese should be bubbly on the top, and the lentils should be piping hot all the way through.
Serve at the table from the oven dish. Works well when accompanied by a green salad with my salad dressing.