Put 1 pound baby spinach in a large microwave-safe bowl, cover and microwave on high until almost fully wilted, 2 to 4 minutes, stirring once. Uncover and set aside until cool enough to handle. Using your hands, squeeze the spinach dry, then roughly chop; set aside. In a large saucepan over medium, cook 3 ounces pancetta (chopped), stirring occasionally, until golden brown and crisp, about 8 minutes. Using a slotted spoon, transfer to a paper towel–lined plate. Add 1 medium shallot (halved and thinly sliced), 1 teaspoon salt and ½ teaspoon pepper to the fat in the pan, then cook over medium, stirring occasionally, until the shallot is light golden brown, about 2 minutes. Stir in 1 cup heavy cream and ⅛ teaspoon grated nutmeg. Bring to a simmer and cook, stirring often, until thickened enough that a spatula drawn through it leaves a brief trail, 4 to 6 minutes. Off heat, stir in 1 ounce finely grated Parmesan cheese (½ cup), 1 tablespoon finely grated lemon zest, the spinach and pancetta. Taste, then season with salt and pepper.