Preheat the oven to 163 C / 325 F. Line a 13 x 9-inch (33 x 23 cm) sheet pan with parchment paper.
Whisk the egg white in a large mixing bowl until frothy and light, about 30 seconds.
Add the mixed nuts to the bowl and toss until every nut is coated in the egg white.
Pour in the melted butter, rosemary, brown sugar, cayenne, smoked paprika and salt. Toss well with a silicone spatula until evenly coated.
Spread the nuts in a single layer on the prepared sheet pan, breaking up any clumps.
Roast for 15 to 18 minutes, stirring once at the 8 minute mark, until fragrant and the edges turn deep golden brown.
Remove from the oven and let the nuts cool on the pan for at least 15 minutes. They'll look soft coming out, that's normal, they crisp as they cool.
Break up any clumps once cooled and transfer to a bowl or jars to serve.