#1 Crumble sausage in a large skillet with the olive oil and brown for 5 minutes. Add leeks and cook 5 additional minutes. Set aside #2 Prepare the Béchamel: Melt butter in a medium saucepan. Add the flour and cook for 3 minutes. Bring the milk to boil in a separate saucepan. Add milk to the butter and flour mixture. Simmer for 5 minutes stirring constantly #3 Combine the sausage and leek mixture with the béchamel sauce. Season mixture with salt and black pepper to taste #4 Preheat oven to 350° F #5 Place one ladle of béchamel sauce on the bottom of a 13 x 9 inch baking dish. Build a total of 4 layers using 4 sheets of lasagne and 1/4 of the sauce mixture for each layer. End with the sauce on the top layer. Sprinkle top layer with Parmigiano cheese #6 Bake covered in foil for 30 minutes. Let rest for 10 minutes before serving