Heat oven to 400 degrees F.   Cook pasta until quite al dente, or 2 minutes less than the suggested cooking time. (Please. It will keep cooking in the sauce, then in the oven and mushy pasta makes me sad.) Reserve 1/2 cup cooking water, then drain pasta.   Heat large sauté pan — if yours is ovenproof, you can even use it as you final baking vessel — over medium heat. Coat with glug of olive oil, and heat oil. Add meat and cook with onion, garlic, oregano, pepper flakes, and salt over medium-high heat for 6 to 8 minutes or until meat is browned; stirring frequently. If you’re using plain ground beef versus sausage meat, you’re going to really want to season this well. Don’t be shy with the salt and pepper.   Add crushed tomatoes and stir to combine. Reduce heat to medium-low and simmer for 5 minutes. Adjust seasonings to taste. If it’s become quite thick, stir in reserved pasta water. Add spinach and cook until wilted, just another minute. Stir in drained pasta and heat together for one minute.   Pour half of pasta mixture into a 9×13-inch baking dish, lasagna pan, or other 3-quart baking vessel (or divide among smaller ones, if you’d like to freeze some off). Sprinkle with half of each cheese. Pour remaining pasta and sauce over, and finish with remaining cheese. Bake in heated oven for 30 minutes.   If you wish, you can run the dish under your broiler for a minute or two for an extra-bronzed lid right before serving