Cut the butter into the oats and sugar with a pastry blender until there are pea-sized clumps. Blend in the cinnamon, salt and chopped nuts. Put the lemon juice in a large bowl and slice the apples into that bowl, tossing after each addition.
Sprinkle on the sugar, flour, cinnamon, salt and nutmeg (if desired). Toss well. Layer half the crumb mixture in the bottom of a buttered crockpot. Press down lightly.
Add all the apples and then another layer of the crumb mixture. Cover and cook on high for 4 hours. Stick a spoon or piece of crumbled aluminum foil under the lid so that steam can escape during cooking. This will allow the top and bottom to "crisp."