Preheat oven to 375 degrees.
Spray a glass pie plate with cooking spray.
Layer potato and squash, alternating layers.
In a small saucepan, over medium heat, melt butter, Add milk and stir until it begins to simmer. Remove about 3 T. hot milk and put in a small bowl. Add flour and whisk together until smooth. Add back to saucepan and, stirring constantly, cook until thickened.
Pour over potatoes and squash. Top with grated Mozzarella cheese.
Bake uncovered for about 40 minutes, until top is slightly browned and vegetables are tender.
Yield: 6 cups