Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish. In a microwave or small saucepan, warm the milk and broth together; set aside.
In a large skillet over medium heat, melt butter. Add flour and cook for 1 minute. Whisk in the warmed milk and broth and bring to a boil stirring constantly. Cook for 2 minutes or until sauce has thickened.
Remove from heat and stir in sour cream, mayonnaise, salt, pepper and garlic powder. Taste and adjust seasonings as needed.
Assemble the lasagna by layering 1 cup of white sauce on the bottom of baking dish. Top with 3 sheets of lasagna, 1 heaping cup chicken, 1 cup cheese and 2 cups of sauce. Repeat layers 2 more times.
Cover with foil and bake for 25 minutes. Remove foil and continue baking for 5-10 minutes or until top is browned and bubbly. Let stand for 10 minutes before cutting.