How to Make It
Cook the manicotti noodles according to the package directions; drain and rinse with cold water.
Heat oven to 425° F. Melt 2 tablespoons of the butter in a large pot over medium-high heat. Add the onion, garlic, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring often, until beginning to soften, 5 minutes. Crush the tomatoes with your hands or a potato masher and add them (with their juices) to the pot. Bring to a simmer and cook, stirring occasionally, until slightly thickened, 10 to 15 minutes.
In a separate large pot, melt the remaining 2 tablespoons of butter over medium heat. Whisk in the flour and cook, stirring, until light golden, 1 to 2 minutes. Slowly whisk in the milk, ½ teaspoon salt, and ¼ teaspoon pepper. Cook, whisking, until the sauce is thick enough to coat the back of a spoon, 2 to 3 minutes. Remove from heat.
Stir together the ricotta, spinach, ½ cup of the Parmesan, the egg, oregano, and ¼ teaspoon each salt and pepper in a large bowl. Spoon into the manicotti. Spread half each of the tomato sauce and cream sauce in the bottom of a 9-by-13-inch or other 3-quart baking dish. Top with the stuffed manicotti, then the remaining tomato sauce, cream sauce, and Parmesan.
Bake until bubbling and lightly browned on top, 20 to 25 minutes. Let cool for 5 minutes before serving.