1Heat oil in large pot over medium. Add prosciutto and cook til crisp, 3 to 4 minutes. Transfer prosciutto to a paper towel lined plate.
2Add leeks, broccoli stems and celery to pot and sweat over medium heat, partially covered, til softened, about 5 minutes. Stir in broth, potatoes, and parsley. bring soup to a boil, reduce heat to medium and simmer til vegetables are tender, 10 to 15 minutes. Puree soup with handheld blender til smooth; stir in blanched broccoli florets.
3Melt butter in pan over medium low heat. Whisk in flour and cook 2 minutes, whisking constantly. Stir in half and half, bring to a simmer and cook til thickened, 2 to 3 minutes. Add mixture to soup along with sherry and lemon juice; season with salt and pepper. Serve soup with prosciutto