Saute onion and celery in margarine about 5 minutes. Blend in flour, dillweed, salt and pepper; cook, stirring constantly until mixture bubbles; slowly stir in milk. Cook, stirring constantly until sauce thickens. Flake tuna, stir in with peas, bring to boiling, stirring often. Pour into a 6 cup casserole; arrange biscuits on top. Bake in 400 oven 20 minutes or until buscuits are brown.