Cook 1 pound raw Italian sausage in a Dutch oven or large pot over medium heat, breaking it up into small pieces with a wooden spoon, until no longer pink, 6 to 8 minutes.

Stir in 1 diced medium yellow onion, 3 minced garlic cloves, 1 teaspoon kosher salt, ¼ teaspoon black pepper, and ¼ teaspoon red pepper flakes if using. Cook, stirring occasionally, until the onion is softened, about 3 minutes. Sprinkle ¼ cup all-purpose flour over the sausage mixture. Cook, stirring constantly, for 1 minute to cook the floury taste out.

Stir in 1 (32-ounce) carton low-sodium chicken broth, scraping the bottom of the pot with a wooden spoon to release any stuck-on bits. Increase the heat to medium-high and bring to a lively simmer.

Reduce the heat to maintain a gentle simmer. Cook for 5 minutes to let the flavors meld.

Stir in 1 (8-ounce) package cheese tortellini and 5 ounces baby spinach. Cook for 1 minute less than the package instructions for the tortellini. Stir in 1 cup half-and-half. Taste and season with more kosher salt and black pepper as needed.