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