In a medium bowl, combine the brown sugar, oats, walnuts ,flour and pumpkin spice mix.
Pour the melted butter over the dry ingredients and mix until the butter coats the dry ingredients. The streusel should clump together when pressed when it's mixed properly.
Use on your favorite desserts.
Freeze any leftovers in an airtight container or bag for up to 3 months and defrost to use.