Make crispy topping: In a food processor or in a large bowl, add the flour, oats, brown sugar, cinnamon, ginger, nutmeg, and kosher salt. Cut in the butter pulsing in the food processor or with a pastry cutter in the bowl until just crumbly.* If using a food processor, pour in the nuts and pulse a few times until finely chopped and well-blended. If using a bowl, add the finely chopped nuts and stir well.