Prep Time: None minutes
Cook Time: 20 minutes
Prick each potato once or twice with fork, place directly on oven rack; bake in preheated 450°F oven until tender, 1 to 1-1/4 hours. Remove from oven; let stand until cool enough to handle.
Meanwhile, in large skillet, melt 2 Tbsp. butter over medium heat. Add leeks, garlic and pinch of salt; reduce heat to medium-low and cook, stirring occasionally, until leeks are very soft, 7 to 8 minutes. Add thyme; cook 10 seconds. Add cream; bring to a boil. Add peas; simmer until peas are barely tender, about 2 minutes. Pour mixture into food processor and let cool slightly. Process until pureed, 45 to 60 seconds; set aside.
When potatoes are cool enough to handle, cut each one in half lengthwise and scoop out, leaving a 1/4-inch shell; transfer potato to bowl.
In large skillet, melt 2 Tbsp. butter over medium heat. Add potato and cook for 2 minutes, stirring occasionally to break up any large chunks; add remaining 4 Tbsp. butter and pepper to taste. Add pea puree and stir until well combined. Stir in lemon zest. Stir in prosciutto, if desired. Add 1 cup cheese and stir. Spoon into potato shells and place on rimmed baking sheet. Top potatoes with remaining cheese. Bake in preheated 375°F oven until heated through and cheese is golden brown, 15 to 20 minutes.