Heat oven to 425 degrees. Season the cauliflower with olive oil, pepper and salt, and roast on a parchment-lined baking sheet until browned and tender, about 20 minutes. Remove from oven and set aside.
Heat a large stock pot and add olive oil. Cook the garlic, anchovies and onions until soft and fragrant, about 3 minutes.
Add the tomatoes, olives, capers, oregano and chile flakes, and bring the sauce to a boil, then reduce to a simmer.
Cook until the tomatoes thicken and sweeten, about 30-40 minutes.
Stir in the spinach and cook until wilted, then stir in the cauliflower.
Season with pepper and salt.
Serve over pasta for a classic Pasta Puttanesca or spoon over fish or chicken.