1To make sauce: Melt butter in sauce pan over medium low heat. Add flour & whisk for about 3 minutes. Whisk in milk & increase heat to medium high. Whisk until comes to simmer & is thick & smooth. Whisk in salt pepper and nutmeg. Pour into a 13x9 baking dish. Set aside.
2Lasagna: Whisk Ricotta, spinach, parmesan, prosciutto, egg, & salt & pepper in medium bowl to blend.
Into a large pot of boiling water add the olive oil and salt. Boil Noodles til aldente, then drain & place noodles on sheet pan coated with olive oil to keep from sticking.
Place lasagna strip down flat & place 3 Tblsp of ricotta mixture evenly over each noodle. Roll up like a jelly roll. Lay seam side down being sure to leave space in between each roll so they are not touching each other. Spoon 1 cup of spaghetti sauce over the rolls & sprinkle mozzarella & parmesan cheese over the top.
3Cover w/foil & Bake @ 450 for 20 minutes & top w/ additional spaghetti sauce.