Preheat oven to 350°.
In a medium bowl, combine 2 cups flour, the baking soda, cinnamon, salt and nutmeg.
In a large bowl, beat softened butter with a mixer on medium 30 seconds.
Add granulated sugar; beat until light and fluffy.
Beat in eggs and vanilla.
Add flour mixture, beating until just combined.
Stir in apples and coarsely chopped pecans.
Line a 13×9-inch baking pan with parchment paper.
Spread batter in prepared pan.
Bake until golden and set in the center, 50 to 60 minutes.
Meanwhile, in a small bowl, stir together brown sugar, toasted pecans, melted butter, the remaining 2 tablespoons flour, the water and oats.
Sprinkle over hot bars.
Bake 5 minutes more.
Cool completely on wire rack.
If desired, use parchment paper to lift bars out of pan.
Place on a cutting board to cut into bars.
