This isn't my MIL's recipe, but I'm sure it's quite similar. This one I copied from the Beverly Lewis' Amish Heritage Cookbook.
Oyster Filling2
12 oz jars oysters, undrained
3 cups milk
3/4 tsp salt
dash pepper
4 cups flat oyster crackers
1/2 cup browned butter
Place oysters, with juice, milk, salt and pepper in saucepan. Heat just to boiling, immediately remove from heat. Butter 4-quart vovered baking dish. Layer oyster crackers and oyster/milk mixture. Crackers will float, gently push them into milk, evenly distributing oysters. Bake, covered, at 325F for one hour. Drizzle with browned butter before serving.