In food processor with steel blade, process basil, garlic, add nuts and salt. With machine running, pour in olive oil in thin stream through feed tube; process till blended. Chill. (May ne frozen at this point). before using, blend in parmesan. makes one cup.
Heat oven to 350. Toasts nuts in oven for 3 minutes; set aside. In ten inch skillet, steam spinach over medium heat for 2 minutes. Remove from heat; drain. Rinse in cold water to stop cooking process. Place spinach in dry towel and squeeze till excess liquid is gone. Chop medium-fine. In small skillet, saute onion in butter over low heat; set aside. In large bowl, combine nuts, spinach, onions, pesto, yolk, pepper and salt.
Heat oven to 375. Place half slice of prosciutto on each chicken breast.Spread filling evenly over prosciutto. Beginning with small end, roll up chicken. Place chicken rolls seam down in greased medium baking dish. Melt butter, remove from hear and add lemon juice; spoon over chicken. Bake 25-30 minutes basting with pan juices. chill. Slice chicken. Lighten remaining pesto with 4 tbsp mayonnaise and serve with chicken.