Preheat oven to 180°.
Blanch cauliflower in boiling water until just hot. Drain and set aside.
Cook pasta until very al dente; drain and mix with cauliflower
In a large saucepan over medium heat, heat oil. Add onion and cook, stirring often, until onion is soft, about 5 minutes.
Stir in garlic, tomato paste and oregano. Cook for two minutes. Season with salt and pepper.
Add crushed tomatoes and bring sauce to a simmer, reduce heat and cook, stirring occasionally, until slightly reduced and flavors have melded, 10 to 15 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.
Add sauce to cauliflower and pasta. Mix well
Spread about half of the pasta mixture into the bottom of a large casserole dish. Cover with half the ricotta mix. Sprinkle half of the mozzarella and Parmesan over the pasta. Top with the rest of the pasta mixture, ricotta mix and sprinkle with the remaining cheeses.
Cover with foil and bake until cheese is bubbling, about 20 minutes. Garnish with more basil before serving.