Preheat your oven to 300°F / 150°C, ensuring the racks are positioned in the center. Line two baking sheets with parchment paper and set them aside.

In a small bowl, mix together the brown sugar, sea salt, curry powder, and lemon zest. Set this mixture aside.

In a large mixing bowl, lightly whisk the egg whites until they are slightly frothy and loosened. Add the walnut halves to the bowl, gently tossing them in the egg whites until they are uniformly coated. This step might take a minute or so to ensure even coverage.

Once the walnuts are well-coated, sprinkle the sugar-spice mixture over them. Toss the walnuts thoroughly to ensure that the sugar and spice mixture evenly adheres to the nuts, taking another minute if necessary.

Evenly spread the coated walnuts across the two prepared baking sheets, arranging them in a single layer. Try to keep the walnuts separated as much as possible to ensure they toast evenly.

Place the baking sheets in the oven and bake for 25-35 minutes, or until the walnuts are a rich golden brown and the coating is hardened and dry. Check to see that the walnuts have a deep golden hue where they contact the pan.

Once baked, allow the walnuts to cool on the baking sheets for about ten minutes. Then, carefully slide the parchment paper, with the walnuts still on it, onto a cool surface to let them cool completely.

After the walnuts have cooled down entirely, store them in an airtight container. These candied walnuts will keep well for several weeks at room temperature, or they can be stored in the freezer for a few months for longer shelf life.