Line a 10×15-inch (25×38-centimeter) baking sheet with parchment paper and grease lightly.

Combine nuts and seeds in a large bowl and set aside.

Combine sugar and water in a medium-size saucepan over medium-high heat. Cook without stirring until the mixture reaches 300 degrees Fahrenheit (150 degrees Celsius) on a candy thermometer. If you don’t have a thermometer, cook until the bubbles become smaller and slower and the syrup turns a deep amber color.

Remove the pan from heat. Stir in the butter or margarine and vanilla. Add orange zest, if using. Sprinkle in the baking soda and stir until the mixture foams.

Quickly fold in the nuts and seeds until evenly coated. Immediately transfer mixture to the prepared pan. Work quickly — the brittle will be thick and stiff rather than pourable.

Using a lightly greased offset spatula or the back of a greased spoon, firmly press and spread the mixture into an even 1/4-inch (1/2-centimeter) layer. It doesn’t need to reach the edges of the pan. If needed, place a sheet of parchment paper over the brittle and press firmly to smooth the surface.

While still hot, sprinkle generously with flaky sea salt and gently press the salt into the surface.

Let cool completely. To speed up the process, place the pan in the freezer for 10–15 minutes once the brittle has stopped bubbling. Break into shards and store in an airtight container.