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1
Shell the peas, keeping both pods and peas. Add the empty pods to the simmering broth and cook for 20 minutes. Strain and discard the pods, returning the now-sweetened broth to a low simmer. The broth should smell distinctly of fresh peas.
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2
In a wide, heavy saucepan set over medium-low heat, warm half the butter with the olive oil. Add the onion and sweat gently, stirring, until completely soft, translucent, and without colour — 8 to 10 minutes. The onion should yield to no resistance when pressed against the pan.
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3
Add the rice and stir to coat in the fat for 1 to 2 minutes, until the grains turn faintly opaque at the edges. Add a ladleful of the pod-infused broth and stir once to combine. From this point, add broth ladleful by ladleful as it is absorbed, but do not stir continuously — this is not a risotto. The rice should cook in a loose, flowing pool of liquid at all times.
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4
After 10 minutes of cooking, add the shelled peas and the pieces of prosciutto crudo together. Stir once gently to distribute. Continue adding broth as needed. This is the step on which the dish lives or dies: the prosciutto must enter the pan raw at this moment — not earlier, or it toughens and turns salty — and the peas must cook only briefly, retaining their sweetness and colour. The surface of the dish should move like a slow wave when the pan is gently shaken.
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5
When the rice is cooked through but retains a faint resistance at the very core — approximately 8 to 10 further minutes — remove the pan from the heat. The consistency at this point must be loose, closer to a thick soup than a risotto; add a final ladle of broth if the mass has tightened.
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6
Add the remaining cold butter in pieces and the grated Parmigiano-Reggiano. Stir briefly and gently — a soft mantecatura that binds without stiffening the dish. Taste and adjust salt. The surface should still flow when the pan is tilted: all'onda, but on the loose side.
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7
Ladle immediately into very hot bowls pre-warmed in an oven or with boiling water. Serve at once. The dish stiffens within minutes of leaving the pan; a cold bowl renders it unservable within 90 seconds.