In a large skillet, heat the olive oil over medium heat. Add the finely diced onion and sauté for about 3-4 minutes until it becomes translucent. Add the minced garlic and cook for an additional minute until fragrant.
Pour in the chicken broth, stirring to combine with the sautéed onions and garlic. Bring the mixture to a gentle simmer.
Add the uncooked orzo pasta and the can of diced tomatoes (with their juices) to the skillet. Sprinkle in the Italian seasonings, kosher salt, and crushed red pepper flakes, if using. Stir everything together.
Cover the skillet and let the mixture simmer for about 10-12 minutes, or until the orzo is al dente and has absorbed most of the liquid. Stir occasionally to prevent sticking.
Once the orzo is cooked, gently fold in the shredded rotisserie chicken and chopped spinach. Cook for an additional 2-3 minutes, or until the spinach has wilted.
Remove the skillet from the heat and stir in the grated Parmesan cheese until melted and creamy. Taste and adjust seasoning if necessary.
