I begin by heating my oven to 375°F (190°C) and moving the rack to the upper third so the top cheese browns nicely. In a medium bowl, I mix ricotta, freshly grated Parmesan, garlic powder, and dried oregano until smooth. I put this cheese mixture aside for later layering.
While the oven heats, I boil the mostaccioli in salted water but stop 2 minutes earlier than the package says, so the pasta is just under-done.
Meanwhile, in the skillet I cook the Italian sausage and chopped onion over medium-high heat until the sausage is browned and cooked through, breaking it apart. I then add garlic and red pepper flakes and cook a short minute until fragrant, draining excess fat.
I return the drained pasta to the large pot, pour in the meat-onion-garlic mix, then add marinara sauce, tossing until everything is coated. Half of this mixture goes into the baking dish. Then I spoon the ricotta layer over that, not worrying about perfect evenness. I top with the remaining pasta mixture, spread it out, then cover with mozzarella. I bake uncovered for about 20 minutes, then switch to broil for a few minutes until the top is bubbling and golden—watching carefully so it doesn’t burn. Finally, I season with salt & pepper, and garnish with fresh basil or parsley if desired.