This recipe is inspired by one by Nigel Slater. It's important
to keep the onion shells intact when removing the insides.
6onions, peeled, keeping root intact40 gmbutter, coarsely chopped50 gmflat mild pancetta, finely diced2field mushrooms, finely chopped25 gmdried porcini, soaked for 5 minutes in warm water, drained, finely chopped2garlic cloves, finely chopped60 gmdouble cream60 gmfinely grated parmesan, plus extra for scattering50 gmcoarse sourdough breadcrumbs1 tspfinely chopped thyme, plus extra sprigs to serveFinelygrated rind of ½ lemon600 mlpouring cream
Cook onions in a large saucepan of simmering salted water over low-medium heat until tender (20-30 minutes). Drain, then, when cool enough to handle, cut 2cm from top of each onion. Remove excess onion from inside tops with a small sharp knife and set tops aside (discard excess onion). Score centres of onion bases, then remove inside of onion with a knife so that a thick layer of onion remains to form a shell (do not pierce the shell; discard two-thirds of the excess onion). Finely chop remaining onion, set aside.
Preheat oven to 180C. Heat butter in a frying pan over medium heat, add reserved chopped onion, pancetta, field mushroom, porcini and garlic and sauté until tender (8-10 minutes). Transfer to a bowl and set aside to cool, then add double cream, parmesan, breadcrumbs, thyme and lemon rind and season to taste. Spoon into onion skins, then place reserved tops on onions (you may have some mixture left over). Place in a baking dish to fit onions snugly. Pour over pouring cream, scatter with thyme sprigs. Bake until golden and warmed through (40-50 minutes; scatter with extra parmesan during last 10 minutes of cooking), serve hot.