Cook lentils and onions in the milk and water until soft - I use a pressure cooker, but not necessary. Meanwhile place sliced tomatoes on bottom of oven-proof dish. I usually flavour them with a little basil, or other herbs. You can use mushrooms instead, or even as well. Elliot suggests celery, which I would dislike, or marrow, which I think would be too watery (I have used courgettes [zucchini] with some success, though).
Anyway, when the lentils/onions are cooked and have absorbed most of the liquid, season with salt, pepper, yeast extract and lemon juice and then either puree or beat until smooth. A food processor is the easiest way of doing this. Pile lentil mixture on top of sliced vegetables, and top with breadcrumb/cheese mixture.
Bake in moderate oven, 350 F, mark 4, 175 C, for 40-45 minutes.
Serve with green salad.