Follow the steps in order for the best result.
Preheat your oven to 350 degrees Fahrenheit (175 degrees Celcius).
Cook the penne pasta according to the package directions until al dente. Once cooked, drain the pasta well and set aside.
In a large skillet or saucepan, melt 1 tablespoon of butter over medium heat. Add the chopped onion and minced garlic and sauté until the onion is tender and translucent, about 5-7 minutes.
Sprinkle the flour over the sautéed onions and garlic. Stir continuously to coat the vegetables with the flour. This creates a roux that will help thicken the sauce.
Gradually pour in the milk, stirring constantly to prevent lumps from forming. Cook over medium-low heat, stirring frequently, until the sauce becomes bubbly and slightly thickened. This should take about 5-10 minutes.
Remove the sauce from the heat. Stir in 1/4 cup of grated Parmesan cheese, Italian seasoning, pepper, and salt. Mix well until the cheese is melted and the seasonings are evenly distributed.
In a large bowl, combine the cooked penne pasta, the prepared sauce, 1 cup of grated Parmesan cheese, diced tomatoes (with their juices), and the squeezed-dry spinach. Mix all ingredients thoroughly to ensure everything is well combined.
Grease a 13x9 inch baking dish with butter or cooking spray. Spoon the pasta mixture into the prepared baking dish, spreading it evenly.
In a small bowl, combine the breadcrumbs, 2 tablespoons of Parmesan cheese, and the remaining 1 tablespoon of butter. Mix together until the butter is evenly distributed and the mixture resembles coarse crumbs.
Sprinkle the breadcrumb mixture evenly over the top of the pasta in the baking dish.
Bake in the preheated oven for 30 minutes, or until the topping is golden brown and the pasta is heated through and bubbly.
Remove from the oven and let it rest for a few minutes before serving. This allows the sauce to set slightly.
Serve hot and enjoy your delicious Penne and Spinach Bake!