Melt the butter in a heavy saucepan, stir in the flour, and cook for a minute or two before gradually adding the milk and stock, stirring continuously to ensure a smooth sauce. Cook for 10 minutes, and remove from the heat. After 10 to 15 minutes, stir into the yoghurt or fromage frais and seasoning.
Slice the aubergine and quarter the peppers, remove the seeds, brush with oil, and grill or bake both vegetables until soft. Trim and slice the leeks, rinse thoroughly and blanch for 5 minutes in boiling water. Thinly slice the potatoes, and parboil them.
Oil or butter a rectangular dish, and spoon some of the sauce over the base. Wet the sheets of lasagne and arrange a layer on top of the sauce. Make the next layer with the aubergine and pepper, slicing the pepper if necessary.
Spoon on more sauce, and cover with a layer of damp lasagne sheets. The second layer of vegetables is the leeks and potatoes. Top this with more sauce, and a final layer of wet lasagne sheets. Spread the rest of the sauce on the lasagne, and on top of that add a generous shaving of Parmesan. Add a little snipped rosemary or thyme to each layer.
Bake for about 30-40 minutes in a preheated oven at 180-200°C/350-400°F/gas mark 4-6.
© 2000 Frances Bissell. All rights reserved.