After I tweeted that I'd made a lentil casserole, three people expressed an interest in the recipe. Posting it here seemed like the best option for sharing it. Please note that quantities are approximate as my usual method for measuring ingredients is "throw some in until it looks right".
Peel and dice one medium onion. Heat a splash of olive oil in a stove-to-oven casserole dish (I love my Le Creuset knock-off, but if you don't have one, just do the first bit in a pan and then transfer it to a glass or crockery casserole) and soften the onion along with a peeled and crushed clove of garlic. When the onion is soft, add about half a cup each of green and red lentils. Give it a good stir, then add enough hot water to cover it all.
Add a pinch of oregano, a teaspoonful of yeast extract, a generous glass of red wine, a few chopped mushrooms, a deseeded and chopped green pepper, and a tablespoonful of tomato puree. Mix everything well and check the liquid level. It needs to be very sloppy, because the lentils will absorb loads of water.
Put the lid on the casserole and pop it in the oven at about 150 degrees Celsius (Doctor Google tells me that's 300 degrees Fahrenheit or Gas Mark 2). Check it after about 45 minutes: if the liquid has disappeared but the lentils are still tough, add extra water and give it a bit longer. When the liquid has gone and the lentils are soft, it's ready to eat. Serve with pasta or just some crusty bread.