Cut the chicken breasts into 1-inch cubes and season evenly with 1 teaspoon salt, 1 teaspoon pepper, and 1 teaspoon garlic powder.
Heat a large skillet over medium-high heat and add 2 tablespoons olive oil.
When the oil is hot, add the chicken in a single layer and cook without moving for 3–4 minutes, then stir and cook another 3–4 minutes until lightly browned; remove chicken to a plate (it may not be fully cooked through).
Lower the heat to medium, add 1 tablespoon butter to the skillet and then add the 16 ounces orzo; stir frequently until the orzo becomes lightly browned and toasty, about 2–3 minutes.
Add 3 cups chicken broth, 1½ cups heavy cream, and ¼ cup fresh lemon juice to the skillet. Stir well, scraping any browned bits from the bottom. Bring to a boil.
Reduce heat to low, cover the skillet, and simmer for 10–12 minutes, stirring once or twice, until the orzo is tender.
Remove the lid and stir in ¼ cup grated Parmesan, the cooked chicken pieces, and 8 ounces fresh spinach. Cook, stirring regularly, until the spinach is wilted and the chicken is cooked through, about 2–3 minutes.
Remove from heat, garnish with lemon zest to taste, and serve immediately.