Saute celery, carrots, and garlic in 3 tablespoons olive oil for 5 minutes. Remove from heat. Stir in milk, spinach, peas, Parmesan, eggs, parsley, Dijon mustard, salt, and flaked halibut. Pour into a 2-quart baking dish. Mix mashed potatoes with
scallions and spread on top of the pie. Bake 40 minutes at 400° F.