Preheat the oven to 350°F (175°C). Heat the olive oil in a large oven-safe sauté pan set over medium-high heat. Add the diced red onions and cook for 3 to 4 minutes, stirring occasionally, until the onions become softened and translucent. Add the minced garlic and cook for an additional 30 seconds.

Stir in the crushed tomatoes and allow the mixture to reach a rapid simmer for 1 to 2 minutes. Reduce the heat to low and let the sauce continue to simmer gently for 5 to 7 minutes, stirring periodically. Incorporate the honey (or sugar), parsley, basil, and oregano. Cook for an additional minute, then season the sauce with salt to taste. Turn off the heat and set aside.

Bring 2 quarts of water to a boil in a medium pot. Season the water generously with salt. Add the penne pasta and cook for 7 to 9 minutes, or until the desired texture is achieved. Drain the pasta and briefly rinse it under cold water to stop the cooking process. Set aside.

Heat the remaining olive oil in a large skillet over medium-high heat. Add the chicken sausage and cook for 5 to 7 minutes, or until fully cooked and lightly browned. Add the prepared marinara sauce and heat for 1 minute.

Add the baby spinach and fold it gently into the sauce. Allow the spinach to wilt, which should take approximately 30 seconds.

Add the cooked pasta to the skillet and stir until evenly combined with the sauce and sausage. Sprinkle the shredded mozzarella cheese over the top. Transfer the skillet to the preheated oven and bake for 10 to 12 minutes, or until the cheese is melted, bubbly, and lightly browned at the edges.

If preferred, the baking step may be skipped by stirring the cheese directly into the hot pasta until fully melted.

Remove the skillet from the oven. Garnish with chopped parsley and season with additional salt and pepper as desired. Serve warm.