Cook onion over medium-high heat 4 to 5 minutes or until soft and translucent. Add garlic; cook an additional minute. Beat in flour with whisk, and cook a minute or two. Slowly beat in stock and milk. Simmer until thickened slightly. Taste for seasoning; add more salt and pepper to taste. Pour sauce into bowl with pasta and chicken. Add lemon peel; stir to coat pasta and chicken. Spread mixture in casserole; sprinkle with green onions. Bake 20 minutes.