Put on dishwashing gloves. Bring 2 L / 2 quarts of water and 1 tbsp salt to a boil in a 4-quart / 3.8 L pot and set a bowl of ice water beside the stove. Use tongs to push the nettles into the water and blanch for 2 minutes, until deep green and limp.

Lift the nettles into the ice water with tongs and chill for 1 minute. Drain, then squeeze them hard in a clean kitchen towel. Blend with 80 ml cold water for 1 to 2 minutes until smooth and vivid green. You'll have about 200 g purée.

Bring the broth to a bare simmer in a 3-quart / 2.8 L saucepan and keep it hot on the lowest heat. In a 28 cm / 11-inch straight-sided sauté pan, warm the olive oil over medium-low heat. Add the onion and a pinch of salt and cook for 7 minutes, stirring, until soft and translucent with no browning.

Raise the heat to medium, add the rice, and toast for 2 minutes, stirring, until the edges turn translucent and the grains smell nutty. Pour in the wine and stir for about 1 minute until it has evaporated and the pan looks nearly dry.

Add the hot broth one ladle (about 120 ml) at a time, stirring often and waiting until each addition is mostly absorbed before adding the next. Keep a lively simmer and continue for 16 minutes. You'll use about 1 L of the broth.

At minute 16, stir in the nettle purée with the next ladle of broth and cook for 2 more minutes, until the rice is al dente: tender outside, slightly firm at the center, with no chalky dot. Don't add the purée earlier, since long heat turns it khaki.

Take the pan off the heat. Add the cold butter, Parmigiano, lemon zest, and lemon juice, then stir hard for 30 seconds until glossy. The risotto should spread slowly across a plate in a wave (all'onda), so loosen it with a splash of hot broth if it stands in a mound. Season with salt and pepper and serve right away in warm bowls.