Annie Bell's Sage and Onion Lasagne

Have ready an ovenproof dish for the lasagne measuring about 30cm x 23cm and 6cm deep, and also roasting dishes for the onions. Preheat the oven to 200C/180C fan/gas 6.
Peel, halve and thinly slice the onions.

Toss with 3 tbsp olive oil and the sliced sage, spread in two roasting tins or dishes in a crowded single layer and roast for 30-40 minutes until lightly coloured, stirring halfway through. Combine all the onions together, stir in the lemon zest and juice, season and leave to cool.

At the same time, peel the garlic and crush to a paste. Melt half the butter in a large pan over a medium heat, stir in half the garlic and half the spinach, and fry, stirring frequently, until the spinach wilts.

Transfer to a bowl, season, then cook the remainder in the same way. Leave to stand for about 15 minutes, then drain in a sieve, pressing out as much liquid as you can. Coarsely chop the spinach.

For the sauce, melt the butter in a medium nonstick pan, add the flour and let the mixture seethe for about a minute. Now, off the heat, gradually incorporate the milk using a wooden spoon, very slowly to begin with.

Add the bay leaf, bring to a simmer, stirring frequently, then turn down the heat and cook on low for 5-10 minutes, stirring from time to time. Remove the bay leaf.

Whisk in the mustard and season with nutmeg and salt. Cover the surface with clingfilm and leave to cool. To assemble, layer the ingredients in your ovenproof dish in the following order. Spread a thin layer of sauce over the base. Arrange a layer of lasagne on top, breaking the sheets to fit.

Now layer over a quarter each of the spinach, onions, gruyère, sauce and finally parmesan. Repeat until all the ingredients are used up.

In total you should have 4 repeats of each of the layers. Toss the whole sage leaves with just enough oil to coat and
scatter over. Bake for 35-45 minutes until golden and bubbling.