Kabocha squash has a pale green rind and light orange flesh. Substitute butternut squash or pumpkin if you have trouble finding kabocha. Prepare the gratin a day ahead and refrigerate; store the breadcrumb topping separately. Bake at 400° for 30 minutes or until heated through.
Arrange squash in a single layer on a baking sheet coated with cooking spray. Bake at 400° for 25 minutes or until the squash is tender.
Melt butter in a large nonstick skillet over medium heat. Add leek and salt; cook 4 minutes or until tender, stirring frequently. Combine milk and flour, stirring well with a whisk. Add milk mixture to pan; bring to a boil. Cook 1 minute or until thick, stirring constantly. Remove from heat. Add cheese, pepper, and nutmeg; stir until cheese melts. Gently stir in squash. Spoon mixture into a 2-quart baking dish coated with cooking spray.
Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 1/4 cups. Add parsley and garlic; pulse twice to combine. Sprinkle the breadcrumb mixture evenly over squash mixture. Bake at 400° for 20 minutes or until golden brown.