1 pound dried spaghetti
4 large eggs
1 pound ricotta cheese
1 1/2 cups whole milk
1 1/2 teaspoons kosher salt
1/4 teaspoon freshly ground black pepper
8 ounces frozen spinach, thawed
1 cup shredded mozzarella cheese
1 cup grated Parmesan cheese (about 2 ounces)
1/4 cup finely chopped fresh basil leaves
2 cups marinara sauce
Arrange a rack in the middle of the oven and heat to 375°F. Meanwhile, bring a large pot of salted water to a boil. Coat the sides and tube of a 12-cup Bundt pan with cooking spray and set aside.
Add the spaghetti to the boiling water and cook until almost tender. Meanwhile, mix the remaining filling ingredients.
Place the eggs, ricotta, milk, salt, and pepper in a large bowl and whisk until smooth and combined. Using your hands, squeeze the excess water from the thawed spinach and add to the egg mixture. Add the mozzarella, Parmesan, and basil, and stir to combine.
When the spaghetti is ready, drain and return to the pot. Immediately add the spinach mixture and mix with tongs to thoroughly combine. Transfer to the Bundt pan and spread into an even layer.
Bake until set and golden-brown around the edges, 45 to 55 minutes. Remove the pan to a wire rack and let sit 5 minutes. Meanwhile, warm the marinara sauce in the microwave or on the stovetop.
Run a thin knife on the edges of the pasta to loosen. Invert a large plate over the Bundt pan. Holding onto the plate and the pan at the same time (use oven mitts or a towel — it's hot!), flip it over. Remove the Bundt pan. Cut the Bundt pasta into slices and serve with the marinara sauce.