Add brown sugar, water, and pumpkin pie spice to a large pan over medium heat. Stir as the sugar starts to melt.
When the mixture starts to boil, add the nuts and stir until the nuts are evenly coated. Stir continuously and continue to cook as the liquid thickens, about 8-9 minutes.
When the sugary mixture has thickened enough that there isn't a lot of excess, turn off the heat and pour the nuts onto a lined baking sheet.
Use the back of a large spoon to spread out the nuts so they can cool evenly. Sprinkle a little flaky salt over all the nuts. Stir the nuts every 5 minutes while they cool to make sure they aren't sticking together.
Transfer to a bowl and enjoy! Keep any leftover nuts in an airtight container.