In a large mixing bowl, combine the cashews, almonds, peanuts, and walnut halves. Give them a good stir to mix them up evenly.
In a saucepan, combine the white sugar, butter, water, salt, black pepper, cumin, and cayenne. Place the saucepan over medium heat and stir until the sugar has dissolved completely and the mixture is bubbling.
Once the sugar mixture is bubbling, remove the saucepan from heat and pour it over the mixed nuts in the bowl. Use a wooden spoon or spatula to toss the nuts, making sure they are evenly coated.
Preheat your oven to 350°F (175°C). Spread the coated nuts onto a baking sheet lined with parchment paper in a single layer. Bake in the preheated oven for about 10 minutes, stirring halfway through.
After 10 minutes, remove the nuts from the oven and let them cool completely on the baking sheet. Once cooled, transfer them to a serving bowl or an airtight container.