First make the sauce by heating the milk and butter together in a saucepan until the butter has melted, then sift over the flour and nutmeg, and whisk furiously until completely smooth. Turn down the heat and let the sauce simmer with barely a bubble for about 10 minutes until nicely thickened. Stir constantly to prevent scorching.
Preheat the oven to 180 °C.
Cook the bay leaves, onion and garlic in the butter and olive oil in a pot for about 5 minutes until the onion is soft and translucent. Add the mushrooms, including any drained soaked ones, and herbs, turn up the heat and cook while stirring until all the moisture has evaporated. Add the stock and tomato concentrate and turn down the heat to simmer. Cook until the sauce is thickened and season well. Remove the bay leaves.
Lightly oil the base of an ovenproof dish and line with instant lasagne sheets. Spread over a quarter of the white sauce, then follow with a third of the mushroom mixture and then a third of the grated Fontina, Gruyère or mozzarella (not the Parmesan). Repeat with lasagne, sauce, mushrooms and cheese until you finally end with a layer of lasagne.
Cover with the remaining white sauce, sprinkle over the Parmesan and bake in the middle of the oven for
45–55 minutes, until golden brown and the sauce is bubbling. It might be wise to place a baking tray underneath the lasagne dish to catch any drips.
Keep warm until ready to serve.
Extracted from Luscious Vegetarian by Sonia Cabano & Jade de Waal (Random House Struik)