In a food processor, add the white sugar and zest and pulse until combined. In a 10-inch skillet over medium heat, toast the walnuts, stirring, until lightly browned and fragrant, 3 to 5 minutes. Transfer to the food processor. Wipe the skillet clean, then add the semolina and toast, stirring, until beginning to brown, 2 to 3 minutes. Reduce heat to medium-low and toast, stirring constantly, until speckled, golden brown and fragrant, another 2 to 3 minutes. Off the heat, stir in the cinnamon, then transfer to a plate and cool to room temperature.