Golden lasagna

I thought this one might come out too sweet and I was, for once, wrong. Carrots cooked to a silky puree make a lasagna that tastes of spring rather than Sunday penance, and doing away with the béchamel means one less pot and one less thing to burn. There is still cheese, naturally, I am not a monster. Serve it with a sharp green salad and accept the compliments.

1. Warm the olive oil in a skillet and add the carrots, onion, garlic, thyme and the teaspoon of salt. Stir to coat everything, then add the vinegar and the vegetable stock. Cook on medium, partially covered, for about 20 minutes, until the carrots are completely soft and the liquid has nearly cooked away.
2. Tip the lot into a blender and blitz to a smooth puree. If it will not catch, add a tablespoon of water to loosen it. Set aside. You want it thick enough to spread, not to pour.
3. Heat the oven to 325°F convection. In a bowl, beat the ricotta smooth with a big spoonful of the carrot puree, 1/4 cup of the parmesan, the egg and the half-teaspoon of salt. That egg is not there for richness, it is there for structure: with no béchamel to bind the layers, the egg is what sets the ricotta as it bakes so a cut slice holds its lines instead of sliding off the spatula.
4. First, ready the pasta. If you are using regular dried sheets, boil them a few at a time for about half the time on the box, since they finish cooking in the oven, then lay them flat on a towel so they do not stick. Oven-ready no-boil sheets go in dry, just stir an extra splash of water into the puree so they have the moisture to soften as they bake. Assemble in a baking dish, a round one is charming. Start with a thin scrape of the plain carrot puree on the base, then a layer of pasta. Over the pasta, spread some of the whipped ricotta and a little more carrot puree, then a sprinkle of parmesan. Pasta again, and repeat. The word lasagna comes from lasanon, the Greek cooking pot the dish was named for long before the noodle turned up.
5. Build until you are nearly at the top, dividing the ricotta and puree so they see you through every layer. On the final layer before the top sheet, scatter the mozzarella. Lay the last sheet of pasta, spread it with a last veil of ricotta or puree, and cover it generously with the remaining parmesan, grating more if it looks shy. Dot with the butter.
6. Bake for about 1 hour, until the top is golden, blistered and bubbling at the edges. If you used no-boil sheets, cover with foil for the first 40 minutes, then uncover so the top can brown. Let it settle for 5 minutes before you cut it, a serrated knife does the tidiest job. Finish each piece with chopped chives and carry on.