Arrange a rack in upper third of oven; preheat to 450°. In a large (at least 12"), high-sided, ovenproof skillet over medium-high heat, heat oil.

Cook sausage, breaking up into large pieces, until browned, 3 to 5 minutes. Stir in red pepper flakes (if using) and cook, stirring, until warmed through, about 1 minute.

Add marinara; season with 1 tsp. salt. Bring to a simmer and cook, stirring occasionally, until slightly thickened, about 10 minutes.

Meanwhile, in a medium bowl, combine cottage cheese, garlic powder, half of Parmesan, and half of parsley, mashing the curds as you stir; season with salt and black pepper.

Add ziti and water to skillet and bring to a simmer. Cook, stirring so pasta doesn’t stick to bottom of skillet, until well combined, about 2 minutes.

Remove skillet from heat, then fold in half of cottage cheese mixture and half of mozzarella, making sure to leave some pockets of cheese throughout.

Dollop pasta with remaining cottage cheese mixture. Sprinkle remaining mozzarella and Parmesan over.

Transfer to oven and bake until pasta is tender and bubbling, and cheese is golden and browned in spots, 30 to 35 minutes.

Let cool 5 minutes. Top with remaining parsley.