Prepare Oven and Pan : Set an oven rack to the middle position and preheat to 275∘F (135∘C). Line the heavy-duty half-sheet pan with parchment paper.
Create the Spice Glaze : In the large mixing bowl, vigorously whisk the egg white and water together until light and foamy, about 45 seconds. Whisk in the maple syrup until just combined.
Combine the Spices : In the separate small bowl, stir together the cinnamon, ginger, sea salt, nutmeg, cayenne, and cloves until uniform.
Form the Coating : Sprinkle the spice mixture into the foamy egg white mixture and whisk until a smooth, fragrant glaze forms.
Coat the Walnuts : Add the walnut halves to the bowl with the glaze. Using the silicone spatula, gently fold and stir until every nut is thoroughly and evenly coated.
Arrange for Roasting : Pour the coated walnuts onto the prepared pan and spread them into a single, even layer. Make sure the nuts are not overcrowded.
Slow-Roast to Perfection : Bake for 25 to 30 minutes, removing the pan every 10 minutes to toss the nuts with a spatula. This ensures they toast evenly on all sides. The nuts are ready when they appear a deep golden-brown and the coating looks matte and dry.
Cool and Crisp : Place the pan on a wire rack and let the walnuts cool completely, about 25 minutes. They will continue to dry and become significantly crispier as their temperature drops.
