In a mixing bowl, dissolve the lemon gelatin dessert and salt in the boiling water. Stir thoroughly until the gelatin is completely dissolved, ensuring there are no granules left. This step ensures a smooth texture in your final salad.

Add the cold water, vinegar, grated or finely chopped onion, and ground black pepper to the gelatin mixture from Step 1. Stir well to combine all flavors evenly. The vinegar and onion give the salad a tangy and savory note.

Place the bowl with the gelatin mixture (from Step 2) in the refrigerator. Let it chill for about 1 hour and 30 minutes, or until the mixture is very thick. It should be ready when pulling a spoon through it leaves a definite mark. For best results, I check it after an hour, so it doesn't over-set before the vegetables are added.

Once the gelatin mixture is thickened (from Step 3), fold in the shredded carrots and thinly sliced celery. Stir gently but thoroughly to distribute the vegetables evenly throughout the gelatin. This step adds a delightful crunch and freshness.

Pour the gelatin and vegetable mixture (from Step 4) into a 3-cup gelatin mold or divide it evenly into six individual molds. Return the molds to the refrigerator and chill until the gelatin is fully set and firm. Patience here pays off, as unmolding is easiest when the salad is well chilled.

To serve, run a thin knife or spatula around the edge of the mold, then invert onto a plate. Serve chilled as a bright, tangy side salad. To make the presentation even prettier, I sometimes garnish with additional thinly sliced celery or a sprig of parsley.