7-8 large stalks celery, about 3/4 lb. (plus inner stalks for garnish)
Large lemon
1 tablespoon black pepper corns
8 green cardamom pods
1 ¼ cups water (12 oz)
1 cup sugar (7 oz)
Seltzer water (about 2 quarts for the entire batch of syrup)
Clean then thinly slice the celery. It should measure about 3 cups when firmly packed into a liquid measuring cup. Set aside. Remove the zest in strips from half of the lemon. Lightly crush the pepper corns and cardamom pods. Set aside.
Bring water and sugar to a boil in a medium sauce pan. Once it reaches a boil, add the sliced celery and return it to a boil for about 1 minute. Stir once or twice in the process. Remove from the heat. Add the lemon zest, pepper, and cardamom to the celery mixture. Stir to combine, then cover the pan and allow to cool to room temperature.
Strain the celery syrup through a fine mesh strainer into a jar or other suitable storage container. Press the celery to extract as much syrup as possible. (Feel free to nibble on some of the candied celery – it’s surprisingly tasty!) Juice the lemon, and add 3 tablespoons of the juice to the syrup. Stir to combine.
Store covered in the refrigerator until ready to use. Mix syrup with seltzer water. A 1 to 4 dilution is a good starting point; adjust to your taste. Serve over ice with a tender inner celery stalk as garnish if desired.
Makes about 2 cups of syrup

Source: Food52