In a medium saucepan, heat the oil and add the onion. Cook over medium heat until softened. Stir in the garlic and cook for 1 minute.
Add the butter and cook until melted. Stir in the flour. Cook the flour mixture until it becomes a deep golden brown.
Slowly whisk in the chicken stock cooking until the mixture begins to thicken. Whisk in the half and half, cream cheese, lemon zest and juice. Stir in the salt, pepper, parmesan and sour cream until smooth.
Add the chicken and poppy seeds to the cream sauce then transfer to a 13 x 9 baking dish.
Combine the melted butter with the cornflakes and crackers. Sprinkle over the casserole. Bake, uncovered, at 350° for 35-45 minutes or until golden brown and bubbling.