If not using rotisserie chicken: Bring a pot of water to a boil, add generous amount of salt and pepper. Once boiling, reduce heat to bring to a simmer and add uncooked chicken breast. Let simmer until cooked through (about 10 minutes). Then, remove chicken breast and shred.
Pour olive oil in a large pot over medium heat.
Once hot, add sweet onion and garlic. Sauté until soft (about 5 minutes).
Then, add chicken broth, chicken, lemon zest, lemon juice, and crushed red pepper to the same pot. Stir, cover, and bring to a boil. Then remove lid and bring to a simmer.
Add in orzo until al dente (about 5 min, read package directions of orzo you are using).
Stir in spinach, green onion, and feta, stirring until the spinach is wilted.
Salt and pepper to taste.
Remove from heat and serve. Add toppings just before serving.