Celery and Fennel Gratin

Name: Celery and Fennel Gratin
Ingredients: Creamy fennel sauce; 1 medium fennel bulb, cut into a small dice; 6 filets of anchovies packed in oil and minced; 3 tablespoons butter; 1/4 teaspoon white pepper; 1/4 teaspoon ground fennel seed; 2 tablespoons flour; 1 1/4 cups half and half; For the gratin; 8 Large stalks celery, cut into 1 1/2 inch pieces; 1 small leek, white and light green part, halved lengthwise and then cut crosswise about 1/2 inch; 4 tablespoons butter; The creamy fennel sauce; 3/4 cup grated parmesan cheese; 1/2 cup panko crumbs