Bring a 6-quart / 5.7L pot of water to a rolling boil and salt it generously, until it tastes like seawater.
Melt the butter in a 12-inch / 30cm skillet over medium heat. Add the shallot and cook for 3 minutes, until soft and translucent but not browned.
Add the diced prosciutto cotto and cook for 2 minutes, until the edges just start to color.
Stir in the peas and cook for 2 minutes, until they're bright green and heated through.
Pour in the cream and lower the heat. Simmer gently for 3 to 4 minutes, until the sauce coats the back of a spoon. Pull the skillet off the heat.
Meanwhile, cook the garganelli in the boiling water 1 minute short of the package time. Reserve 1 cup / 240 ml of pasta water, then drain the pasta.
Add the drained garganelli to the skillet with a splash of the reserved pasta water. Toss over low heat for 1 minute to finish cooking the pasta in the sauce.
Off the heat, stir in the grated Parmigiano a little at a time, adding more pasta water as needed until the sauce turns glossy and clings to the pasta.
Grate in the nutmeg and add black pepper. Toss once more and taste for salt.
Plate immediately and top with extra Parmigiano and a final crack of pepper.