Before starting, preheat the broiler to high.Wash and dry all produce.Heat Guide for Step 1: 1/8 tsp (1/4 tsp) mild, 1/4 tsp (1/2 tsp) medium and 1/2 tsp (1 tsp) spicy! Add 10 cups water and 2 tsp salt to a large pot (use same for 4 ppl). Cover and bring to a boil over high heat. Meanwhile, peel, then cut onion into 1/4-inch pieces. Peel, then mince or grate garlic. Roughly chop parsley.Roughly chop spinach. (TIP: Place in a bowl, then chop using kitchen shears or scissors!)Add half the garlic salt and 1/4 tsp (1/2 tsp) chili flakes to the container with ricotta. (NOTE: Reference heat guide.) Season with pepper, then stir to combine.

Add rigatoni to the boiling water. Cook uncovered, stirring occasionally, until tender, 10-13 min.Reserve 1/4 cup (1/2 cup) pasta water, then drain rigatoni.

Meanwhile, heat a medium oven-proof pan (large oven-proof pan for 4 ppl) over medium heat.When the pan is hot, add 1 tbsp (2 tbsp) oil, then onions. Season with salt and pepper. Cook, stirring occasionally, until tender, 4-5 min.

Add garlic and remaining garlic salt to the pan with onions, then season with pepper. Cook, stirring often, until fragrant, 30 sec. Add crushed tomatoes, broth concentrate and 1/2 tsp (1 tsp) sugar. Bring to a simmer, then reduce heat to medium-low. Cook, stirring occasionally, until sauce thickens slightly, 6-8 min.

Add rigatoni, spinach, reserved pasta water and half the mozzarella to the pan with sauce.Season with salt and pepper, to taste, then toss to combine. (NOTE: If you don't have an oven-proof pan, transfer to an 8x8-inch baking dish for 2 ppl, or 9x13-inch baking dish for 4 ppl.)Sprinkle with remaining mozzarella, then dollop seasoned ricotta over top.Broil in the middle of the oven until cheese is golden-brown, 3-5 min.

Let rigatoni al forno cool, 5 min.Divide between plates.Sprinkle parsley over top.Sprinkle with remaining chili flakes, if desired.