• Preheat oven to 375° Fahrenheit. Lightly grease 9x13 baking dish, coating bottom and sides well, then set dish aside.

For the Italian Meat Sauce

• Heat large skillet over medium heat. When pan is hot, add olive oil and swirl pan to distribute oil evenly across entire surface. Continue heating pan until oil is hot and shimmery.
• When oil is hot, add diced onion to skillet. Sauté onion, stirring often, 3 to 5 minutes or until onion begins to soften.
  
• Add Italian sausage to skillet and cook 8 to 10 minutes, breaking up sausage with back of wooden spoon, until sausage is crumbled and mostly browned.
  
• When sausage is ready, drain any excess fat from skillet and discard. Return skillet to heat and add marinara sauce, diced tomatoes, Italian seasoning, rosemary, red pepper flakes, salt, and pepper.
  
• Stir well to fully incorporate ingredients, then simmer mixture 10 to 15 minutes or until sauce has thickened. Stir occasionally to prevent sticking or burning.
  

For the Pasta

• While sausage mixture simmers, bring large pot of water to boil over high heat. Once water begins to boil rapidly, add salt and stir to incorporate.
• Allow water to return to full boil, then add dry ziti. Boil ziti 1 to 2 minutes less than indicated in package directions.
• Once pasta is just barely al dente, immediately pour water and noodles into colander. Drain well, then set aside.
  

For the Baked Ziti

• Add ricotta, 1 cup mozzarella, and ¼ cup parmesan to medium mixing bowl. Stir well to fully incorporate ingredients, then set bowl aside.
  
• Place 1 ½ cups of meat sauce to bottom of prepared baking dish. Spread sauce out evenly to cover.
  
• Place 50% of boiled ziti on top of sauce in one even layer.
  
• Top ziti with 50% of ricotta mixture. Spread ricotta out evenly across top of pasta to cover completely.
  
• Repeat layers with remaining sauce, pasta, and ricotta mixture, then sprinkle remaining mozzarella cheese evenly across top of dish.
  
• Place baking dish in preheated oven. Bake, uncovered, 25 to 30 minutes or until cheese has melted completely and top layer of mozzarella is golden and bubbly.
• When baked ziti is ready, carefully remove baking dish from oven. Let stand 3 to 5 minutes, then divide dish between serving plates. Sprinkle chopped parsley over individual servings if desired, then serve warm with preferred sides.