Drop the dry ziti rigati into salted water and cook until just tender. The pale rigid tubes turn glossy and plump and the shape softens while still holding its hollow center. By the end the ziti rigati looks fuller and slightly curved with a smoother surface and a hydrated texture. This is the first big visual shift because the pasta moves from dry and chalky to tender and flexible.

Cook the mild Italian sausage with diced yellow onion and minced garlic cloves until the meat breaks into uneven crumbles and the onion turns soft. Then stir in crushed San Marzano tomatoes and tomato paste with the dried herbs and let everything simmer until the sauce thickens. The mixture changes from separate raw pieces into a deep red sauce with browned sausage bits throughout. It looks looser at first and then becomes richer and more cohesive as the tomatoes darken and the surface turns softly glossy.

Stir the whole milk ricotta with the large egg and some finely grated Parmesan cheese and chopped fresh parsley until the filling becomes smooth yet softly textured. The separate ingredients disappear into one pale mixture with green flecks scattered through it. This step creates a lighter creamy layer that contrasts with the red sauce. Instead of loose curds and streaks you now have a thick spoonable filling that will settle into uneven pockets between the pasta.

Spread some sausage tomato sauce into the baking dish and add the cooked ziti rigati then spoon in the ricotta filling and scatter shredded low moisture mozzarella and more sauce between the layers. The dish changes from separate components into a structured casserole with red and white patches and pasta peeking through. Nothing looks perfectly even here and that is exactly right. The layers should look loosely assembled with sauce tucked between the pasta and cheese scattered in irregular pockets rather than pressed into neat lines.

Bake the layered ziti until the shredded low moisture mozzarella melts into uneven blankets and the top develops browned spots. The sauce bubbles up around the edges and the whole dish settles into a more unified bake with soft pasta and creamy pockets inside. Scoop one portion onto a white plate so the layers open slightly and show the red sauce and ricotta and melted cheese. The finished serving should look rustic and rich with stretched cheese and browned patches and a few scattered bits of chopped fresh parsley on top.