Prepare Bechamel sauce: In a medium pot, melt butter over medium heat. When butter has completely melted, add the flour and whisk until smooth, about 2 minutes. Gradually add the milk, whisking constantly to prevent any lumps from forming. Add salt, nutmeg and pepper and continue to simmer and whisk over medium heat until the sauce is thick, smooth and creamy, about 5 - 8 minutes. The sauce should be thick enough to coat the back of wooden spoon. Remove from heat and check for seasoning. Set aside.
Preheat oven to 375º F. Line a large baking sheet with aluminum foil.
Spray with butter a 13 x 9-inch baking dish. Spread 1/2 cup of the bechamel sauce into the bottom.
Assemble Pasticho: Arrange the 3 pasta sheets side by side, covering the bottom of the baking dish. Evenly spread a layer of bolognese sauce, about 2 cups. Spread 1 1/2 cup of the bechamel sauce. Sprinkle 1/2 cup of parmesan cheese on top. Arrange another 3 layer of pasta sheet, bolognese sauce, bechamel sauce and parmesan cheese. Repeat. Arrange the final layer of pasta sheets and top with remaining béchamel and Parmesan cheese.
Cover the lasagna dish with aluminum foil and place it on the baking sheet. Bake on the middle rack of the oven for 45 - 50 minutes, or until top is bubbling and the pasta is tender. Remove cover and continue to bake for about 15 minutes or until the cheese on top becomes golden.