Use La Quercia Prosciutto or Speck - a package of the pre-sliced is about right.
The poppy seeds add a lot of visual appeal and a mild bit of crunch, so don't leave them out. The acidity was perfect, even with the wine, the lemon juice, and the buttermilk. I had to change things a bit, since I had no heavy cream - I just doubled the buttermilk and added a bit more butter. It was less creamy, but still very flavorful. I also added the scallions all at once, since I wanted to mitigate the raw onion effect. I did think that there was not quite enough sauce for the suggested quantity of pasta, so I increased the amount of prosciutto and green onion.
2 tablespoons unsalted butter
2 tablespoons extra-virgin olive oil
2 large shallots, thinly sliced
Salt and pepper to taste
2 tablespoons poppy seeds
1/2 cup white wine
1/2 cup buttermilk
1/2 cup heavy cream (or use only buttermilk for a less rich dish)
16 ounces tagliatelle
2 to 4 ounces prosciutto, thinly sliced and cut in 1 inch pieces
1/2 cup or more finely grated Parmigiano Reggiano, plus more for garnish
1 bunch scallions (about 6), cut in 1/2 inch slices
Melt butter in 12" skillet, add shallots, salt and pepper and cook over medium heat until soft, about 3 minutes. Add the poppy seeds and cooks until they are fragrant and shallots begin to caramelize a little. Add wine and reduce until almost gone. Then add buttermilk and cream (if using) and reduce slightly.
Cook pasta until al dente (the tagliatelle I used took only two minutes). Drain, reserving 1 cup of cooking water, and add to sauce in skillet. Add the prosciutto, grated parmigiano, the scallions, and the lemon juice. Toss to combine and add the reserved cooking water if necessary. Check seasoning and add salt and pepper if necessary. Serve with more cheese if you want to.
And here's a happy customer at the end. He really liked it!