Preheat oven to 350 degrees. Line an 11x7-inch (or 9x9-inch) baking pan with parchment paper. Beat the butter and sugar at medium speed of an electric mixer until light and fluffy. Slowly beat in the flour and salt until a soft dough forms. Press the dough into the bottom of the prepared baking dish. Bake for 10 minutes, or until the dough is set but not brown.
In a large skillet melt the butter and brown sugar. Add the apples and cook, stirring occasionally, over medium high heat until softened. Stir the water, cinnamon and nutmeg into the apple mixture and cook another 5 minutes or until the apples are caramelized, tender and the liquid is absorbed.
Set aside to cool while preparing the topping.
In a medium mixing bowl combine the oats, flour, brown sugar, cinnamon, soda and salt. Using a pastry blender or two knives, cut in the butter until the mixture resemble coarse meal. Stir in the walnuts and press the topping into clumps.
Spread the apples over the crust and sprinkle with the topping. Press the topping down into the apples to make an even layer. Bake about 40 minutes or until the topping is golden brown. Cool completely then cut into 2-inch bars.