Cut each apple in half along its equator. Using a melon baller, cut out each side of the core, creating a rounded hole. Rub exposed apple flesh with lemon juice. Place 1 tablespoon of jam into each hole. For the topping: in a small bowl mix together flour, butter, brown sugar, oats, cinnamon and salt. Press this mixture on the top of each apple, covering jam. Place in a baking dish filled with about a 1/4-inch of water. Bake until top is golden brown and apple is tender, about 35 to 40 minutes.