Heat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Warm olive oil in a large oven-safe skillet over medium-high heat. Brown the sausage, breaking it up.
Stir in onion and bell pepper. Cook until softened, about 4 minutes.
Add garlic, Italian seasoning, and red pepper flakes. Cook 30 seconds until fragrant.
Mix in uncooked rice and toss to coat. Toast 1 minute, stirring.
Pour in chicken broth and crushed tomatoes. Scrape up any browned bits and bring to a simmer.
Season with salt and black pepper. Fold in spinach, if using.
Transfer to the prepared dish (or keep in the oven-safe skillet). Cover tightly with foil and bake 25 minutes.
Uncover, stir, and sprinkle mozzarella and Parmesan over the top.
Bake uncovered until rice is tender and cheese is browned in spots, 10–15 minutes. Rest 5 minutes, then garnish with parsley.