Preheat the oven to 325°F (163°C). Line a rimmed baking sheet with parchment paper.
Scatter the pine nuts on the baking sheet and slide it in the oven just until the pine nuts are barely golden, 4 to 6 minutes. Dump the pine nuts into a bowl and add the mixed seeds. Keep the baking sheet handy.
In a heavy-bottomed saucepan over medium heat, combine the sugar and water and wait until the sugar dissolves.
Add the butter and honey or corn syrup and stir until the butter melts. Carefully attach the sugar thermometer to the pan and bring to a boil. Continue to cook over medium heat, without stirring, until the mixture is a dark amber and reaches 349°F (176°C), 10 to 15 minutes. (It can feel like an eternity while you’re waiting for the mixture to reach this temperature, but keep in mind, once it goes over 284°F (140°C), it will shoot up pretty quickly. Consider yourself warned and be prepared to move swiftly when the moment arrives. You may even want to pull the pan from the heat a couple degrees before 349°F as the residual heat will kick the temperature up even after it’s off the heat.)
When the mixture in the pan reaches the proper temperature, remove the pan from the heat, add the salt and orange flower water (if using), and stir to combine.
Add the nuts and seeds and mix vigorously so they’re evenly coated with caramel. Working quickly, scoop the mixture onto the parchment-lined baking sheet and use the back of a spoon or offset spatula to spread the mixture evenly. Let cool completely until it’s hardened, about 1 hour.
Break the brittle into pieces and serve.