Alfredo sauce is typically loaded with saturated fat from butter and cream. When you use ground walnuts as the base for Alfredo sauce, you not only improve the nutritional profile, you also add great flavor.
Place walnuts and water in food processor. Purée until very smooth. Set aside.
In a medium saucepan, whisk together milk and flour. Add garlic, salt, and pepper. Bring to a simmer, stirring until slightly thickened and smooth, about 3 minutes. Stir in walnut mixture; simmer and stir over low heat until thickened, 1-2 minutes.
Meanwhile, cook pasta according to package directions, adding peas during last 2 minutes of cooking time. Drain.
Combine pasta, peas, Parmigiano-Reggiano, and sauce. Stir over low heat just until heated through and pasta is evenly coated with sauce. Top each serving with a few strips of prosciutto.