In a medium sauce pot, bring 1/2 cup sugar and 1 cup of water to a boil. Stir briefly to dissolve the sugar, then add the cranberries. Allow the pot to come back up to a boil, turn the heat down to medium-low, place a lid on top, and let it simmer for about 10 minutes. (the cranberries will burst as the cook, so don't forget the lid).
Meanwhile, stir one envelope of plain gelatin into 3 tablespoons of water. Let the mixture sit until the gelatin has absorbed the water and formed a thick jelly-like texture (about 5 minutes).
Once the cranberries have simmered for 10 minutes, stir in the softened gelatin, lemon juice, and cinnamon. Place the pot in the refrigerator to cool.
While the cranberry mixture is cooling, finely dice the celery and apple. Stir together the celery, apple, and walnuts in a large bowl. When the cranberry mixture has cooled a little (it's okay if it's warm, just not piping hot), pour it into the bowl with the celery, apple, and walnuts. Stir to combine, then return to the refrigerator to cool completely (about 2 hours). Serve chilled.