In a large nonstick skillet, melt the butter over medium. Add the prosciutto in a single layer and cook, flipping halfway through, until golden and crisp, 2 to 4 minutes. Press occasionally with a spatula to ensure even crisping and reducing the heat as necessary if the fat begins to smoke. Transfer the prosciutto to a plate, leaving the fat in the pan.

To the skillet, add the shallot and cook over medium until softened, 2 to 4 minutes, adding about ½ tablespoon butter if the pan is dry. Add the tortellini, peas, chicken broth and bring to a boil. Once boiling begins, add the Parmesan little by little, stirring to incorporate the cheese after each addition. Simmer over medium-high, stirring gently but constantly, until the pasta and peas are tender, 5-7 minutes. (The sauce will  be light, but it will thicken a bit more as it cools.) Turn off the heat and stir in the lemon zest and juice. Season to taste with salt and pepper, add nutmeg if desired. Crumble the prosciutto on top.