Sausage lasagna spread thin for the crust

Show this to an Italian grandmother and she will cross herself. Let her. Layering is the fiddly part of a lasagna and the part nobody tastes, so stir the lot together instead, spread it thin on a sheet pan and let the oven brown the whole top. She will call it a sacrilege, and she will still be back for another square before the candles burn down.

1. Heat the oven to 425°F and grease a straight-sided 12-by-17-inch sheet pan with 1 tbsp of the oil; a sloped rim lets it spill over. Put a large pot of well-salted water on to boil.
2. Heat 1 tbsp of oil in a large skillet over medium. Add the garlic, ½ tsp of the salt and the pinch of pepper flakes and cook 1 minute. Add the spinach and cook until it collapses and its water has cooked off. Snip it into pieces in the pan with scissors, then scrape it into a bowl and stir in the ricotta.
3. Add the last 2 tbsp of oil to the same skillet over medium-high. Put in the sausage and leave it alone for 3 minutes so it browns, then break it up and cook 5 minutes more. Add the onion and cook 3 minutes.
4. Stir in the passata, the crushed tomatoes and the remaining ½ tsp of salt. Swirl a couple of tablespoons of water round the bottle and tip that in too; it is sauce, and you paid for it.
5. Boil the pasta for 4 minutes only. It will be firm, which is correct. Scoop out ½ cup of the water, drain and tip the pasta back into its pot, which is the only thing in the kitchen big enough. Add the sauce, the pasta water, 1 cup of the mozzarella and ½ cup of the Parmesan and stir.
6. Spread it over the sheet pan. Dollop the spinach ricotta across the top, then scatter over the remaining 1½ cups of mozzarella and 1 cup of Parmesan.
7. Bake uncovered 30 minutes, until it bubbles and the edges go dark gold. Give it 5 minutes before you cut it, or the squares slide.