Trim the leeks, discarding the roots and the tough tips of the leaves. Slice the leeks into rings about as thick as a pencil, then pile into a colander and wash very thoroughly in cold water.
In a large saucepan, melt the butter, stir in the leeks and cover the surface with a piece of greaseproof paper or baking parchment. This will help the leeks to cook in their own steam without browning. Cover with a lid and leave to simmer gently.
Peel and finely dice the carrots and add to the leeks, then finely dice and stir in the celery. Continue cooking for about 10 minutes, until the vegetables are tender.
Cook the lentils in boiling, lightly salted water for about 15 minutes, until tender but not soft. Drain and set aside.
Stir in the vegetable stock and the miso paste and continue cooking for about 10 minutes.
Wash the greens, roll them up and shred into ribbons. I like to keep them quite wide, like pappardelle. Stir them into the soup and continue cooking for 3 or 4 minutes until the greens have relaxed. Finely grate the lemon and stir into the leek soup and check the seasoning, adding salt and black pepper.
Ladle the soup into deep bowls, the lentils scattered over the surface.
Remove the core from the apples with an apple corer or small, sharp knife. Score the apples around their horizon, cutting through the skin, but not too deeply into the flesh. Place the apples in a baking dish, but make sure they are not quite touching. Allow them a little room in which to puff up.
Make the filling by crumbling the sponge cake crumbs into a mixing bowl. Then add the mixed spice, honey, golden raisins and cranberries to the crumbs. Warm the redcurrant jelly in a small saucepan until it melts, then stir into the crumb mixture.
Stuff the hollows of the apples with the fruit and crumb filling using a teaspoon, scattering any surplus around the base of the fruit. Pour the orange juice into the baking dish, then bake for about 50 minutes until the skins have split open. The apples should be soft and frothy, but not collapsed, so keep an eye on them.
Serve with the juice in the baking dish spooned over them or with a jug of cold double cream.