Preheat oven to 400°F. Gather all ingredients.
Heat 1 tablespoon oil in a large skillet over medium-high. Add chicken to pan and sprinkle with ¾ teaspoon salt and ½ teaspoon pepper; cook until browned, stirring occasionally, 6 to 8 minutes.
Remove chicken from pan. (It’s OK if chicken isn’t completely cooked through.) Add remaining 1 tablespoon oil to pan. Add onion and garlic; cook until onion just starts to soften, 2 to 3 minutes, scraping bottom of pan to loosen any browned bits.
While chicken browns, coat a 13 x 9–inch baking dish with cooking spray. Add chicken stock, cream, Italian seasoning, and remaining ¾ teaspoon salt and ½ teaspoon pepper and stir gently to combine.
Add sun-dried tomatoes, orzo, browned chicken, and onion mixture; stir gently to combine.
Cover dish with foil and bake at 400°F for 35 minutes.
Remove dish from oven (leave oven on). Stir in cheese, then gradually stir in spinach.
Re-cover dish and bake until pasta is done and spinach is wilted, 10 minutes.
