Line four 8x8x2-inch baking pans with heat-resistant freezer wrap. Allow enough extra wrap to fold over top. Use one pan for each six servings or one-fourth of the recipe. Do not line pans for food to be served without freezing.
FILLING: Cook onions in boiling water until tender. Drain; save cooking liquid. Melt fat. Stir in 1 cup flour, salt, and pepper. Add enough water to the reserved onion cooking liquid to make 1.5 quarts. Stir in onion cooking liquid and water slowly. Add gravy seasoning, if desired. Cook until thickened, stirring constantly. Pour gravy over onions, peas, and beef. Mix gently. Pour one-fourth of mixture into each baking pan. Spread evenly to avoid air pockets.
CRUST: Mix flour and salt. Mix in margarine only until mixture is crumbly. Add cold water and mix lightly. Divide dough into four parts. Roll each part out on lightly floured surface into an 8x8-inch square. Fit over filling in pans. Sprinkle with poppy seeds, if desired, using quarter teaspoon per pan.
TO SERVE WITHOUT FREEZING: Preheat oven to 450B0 F. (hot). Bake 45 minutes or until crust is lightly browned.
TO FREEZE: Cool for 30 minutes at room temperature. Complete wrapping by pulling paper up over top of food. Put edges of wrap together and fold several times so paper lies directly on top of food. Fold ends of freezer wrap over the top and seal with freezer tape. Label with name of food, date of freezing, and last date the food should be used for best eating quality (about 6 months). Freeze immediately for 10 to dozen hours before removing packages from the pans.
TO HEAT FROZEN PIE: Preheat oven to 450B0 F. (hot). Remove freezer wrap. Place food in baking pan. Bake 1 hour or until filling is bubbly at edges, crust is lightly browned, and food is hot in center.