These Chicken Gratins can be prepared 3 days in advance and can also be frozen.
Put the carrots and celery in a medium saucepan,, cover with cold water, bring to a boil and cook 15 Minutes. Add the peas and cook 5 Minutes. Drain the vegetables and rinse under cold water. Set aside.
Meanwhile, in a medium saucepan over low heat, melt the butter. Add the onions and cook 5 Minutes stirring occasionally with a wooden spoon. Remove the pan from the heat and stir in the flour. Return to the heat and cook 1 minute stirring constantly. Remove the saucepan from the heat, gradually whisk in the chicken broth and the milk. Return the saucepan to the heat, bring to a boil stirring constantly, reduce heat and simmer for about 5 minutes stirring occasionally with the spoon, until the sauce thickens. Add salt and pepper.
Set the oven rack in the lower third of the oven. Preheat the oven to 375°F.
Combine vegetables, chicken and sauce and pour into four (2 cups) casserole dishes.
In a bowl, mix the mozzarella and breadcrumbs and sprinkle this mixture over the gratins.
Bake 30 Minutes or until cheese is lightly browned. Let stand 5 to 10 minutes before serving.