Heat the oven to 200C (180C fan)/390F/gas 6. Wash the chard and rip the leaves off the stems. Cut the stems into 1cm-thick slices, then cut the leaves into roughly 2-3cm-wide ribbons (they will wilt down and shrink in the sauce).
Warm the olive oil in your largest casserole over a medium heat, then fry the pancetta for five to six minutes, until it begins to release some of its fat and begins to colour around the edges. Add the garlic and thyme, and cook for two minutes more, until it is soft and smelling fragrant. Add the leeks and chard stems, season well and cook, stirring often, for 10 minutes, until everything is soft.
Stir in the chard leaves, still dripping with water, cover the pan and steam for three to five minutes, until the leaves have wilted. Stir in half the ricotta, 250ml water and season to taste.
Combine the remaining ricotta with the egg yolks, 70g parmesan and creme fraiche in a bowl and season generously.
Spread a quarter of the chard mix over the bottom of a very large baking dish. Now layer with uncooked lasagne sheets, the ricotta mix and more chard until you have three to four layers, finishing with the ricotta mix. Sprinkle over the remaining 30g parmesan, drizzle with a little oil, cover with foil, and bake for 30 minutes. Remove the foil and bake for another 20 minutes, until golden on top.
Leave to cool slightly before serving with a lovely green salad.