Preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper or a silicone baking mat.
In a large mixing bowl, combine the almonds, cashews, pecans, and walnuts.
In a small bowl, whisk together the melted butter, brown sugar, smoked paprika, cumin, cayenne pepper, salt, and black pepper until well blended.
Pour the spice mixture over the nuts and toss until all the nuts are evenly coated.
Spread the coated nuts in a single layer on the prepared baking sheet. Bake in the preheated oven for 12-15 minutes, stirring once halfway through, until the nuts are fragrant and lightly toasted. Watch closely to prevent burning.
Remove from the oven and let the nuts cool completely on the baking sheet (they’ll crisp up as they cool). Sprinkle with chopped fresh rosemary if desired. Serve in a bowl as a snack or appetizer. Enjoy your spiced mixed nuts!