Vegetarian Baked Ziti

Preheat oven to 350°. In a large pot of boiling salted water, cook pasta until very al dente; drain. In a large saucepan over medium heat, heat oil. Add onion and cook, stirring often, until onion is soft, about 5 minutes minutes.  Stir in garlic, tomato paste, and oregano and cook 2 minutes more, until slightly darkened. Add crushed tomatoes and season with salt and pepper. Bring sauce to a simmer, reduce heat and cook, stirring occasionally, until slightly reduced and flavors have melded, 15 to 20 minutes. Remove from heat and stir in basil. While sauce is reducing, stir to combine combine ricotta, spinach, lemon zest, and crushed red pepper flakes in a medium bowl. In a large bowl, combine sauce and pasta. Fold in ricotta mixture, leaving large clumps. Spread about half of the pasta mixture into the bottom of a large casserole dish. Sprinkle half of the mozzarella and Parmesan over the pasta. Top with the rest of the pasta mixture and sprinkle with the remaining cheeses. Cover with foil and bake until cheese is bubbling, about 20 to 25 minutes. Garnish with more basil before serving.