In a large saucepan, warm oil over medium heat. Place quartered mushrooms & sliced leeks in the saucepan. Season with salt & pepper. Saute until veggies are softened & most of the moisture has evaporated, approximately 7-9 minutes.
Remove saucepan from heat & mix in mozzarella, 1/2 cup breadcrumbs & 1/4 cup parmesan cheese. After combining, transfer the mixture into a lightly greased 8 X 8-inch baking dish, ensuring even distribution.
In a separate small bowl, combine finely chopped parsley, melted butter, remaining parmesan & breadcrumbs. Once mixed, evenly sprinkle this topping over the mushroom mixture.
Bake until breadcrumb topping is golden, about 25-30 minutes.