To a medium bowl, add flour, oats, brown sugar, walnuts, butter, ground cinnamon and a pinch of salt. Use your hands and/or a pastry cutter to combine ingredients. Push it through your fingers to create the chunky, crumbly texture. Cover the apple mixture completely with the crumble topping.
1/2 cup all-purpose flour, 1/2 cup old-fashioned oats, 1/3 cup light brown sugar, 1/3 cup walnuts, chopped, 5 tbsps. butter, room temperature, 1/2 tsp. ground cinnamon, 1/8 tsp. table salt