Preheat oven to 350 degrees F. First, make the marinara sauce. In a saucepan heat oil. Add onion and cook for 3 minutes. Add garlic and cook for 2 minutes more. Add tomatoes, pinch of sugar, salt and pepper and simmer, covered, stirring occasionally, for 20 minutes. Puree the mixture through the fine disk of a food mill and then return to the saucepan. Reduce for 15 minutes. Meanwhile, make your filling. In a bowl mix together ricotta cheese, Parmesan cheese, egg yolk, minced basil, parsley, nutmeg, salt and pepper to taste. Fill your manicotti tubes with the filling mixture. When all the tubes are filled place them in a baking dish. Spoon marinara sauce over the tubes and sprinkle with mozzarella cheese. (Can be refrigerated for several hours or overnight at this point or may be frozen.) When ready to cook, place in the oven and bake, covered for 20 minutes. Uncover and bake for 5 to 10 minutes longer, or until bubbling and heated through. Serve each portion with a sprig of fresh basil.