Preheat the oven to 350°F/177°C. Grease a 13 x 9- x 13 inch/23 33 x 33 23 cm pan with nonstick spray and line it with parchment paper, leaving the paper ends protruding to so that it hangs overhang on two 2 opposite sides.
In the work bowl of a food processor, combine 1 1/2 cups/150 g of the oats, and the baking soda, baking powder, cinnamon, nutmeg, and salt. Process until the oats are broken down. Add the eggs, applesauce, brown sugar, and melted butter and process just until the ingredients are well-mixed.
Add the remaining 1 1/2 cups/150 g of rolled oats and the toasted nuts, and pulse 5 to or 6 times, until the mixture is just combined.
Spread the batter evenly in the prepared pan and bake for 30 to 35 minutes, or until the tops are a golden brown, the corners have begun to pull away from the sides of the pan, and a toothpick inserted in the middle comes out mostly clean.
Remove them the oat slab from the oven and let them it cool completely before removing from the pan. Once it is cool, use the overhanging parchment paper to lift the bars slab out of the pan. Slice into 12 equal bars with a serrated knife.
Store in an airtight container at room temperature for up to a week. For longer storage, wrap the bars individually, tuck them into a ziptop resealable plastic bag, and freeze.