Harvest and prepare the flowers: Pick the elderflower heads on a dry, sunny day when they’re most fragrant. Gently shake off insects and remove any browned blossoms. Avoid washing them, as that can rinse away the aroma.
Make the sugar syrup: In a large pot, bring the water, sugar, and citric acid to a boil. Turn off heat.
Place the elderflowers in the syrup along with the lemon slices. Stir gently to submerge everything.
Cover the container with a lid let the mixture steep in a cool place for 24–72 hours. Stir once or twice if you like.
Strain the cordial through a sieve, cheesecloth, or nut milk bag into a clean pot.
For a clearer cordial, strain again through a fine mesh sieve.
Bring the strained cordial to a boil in a large pot.
Carefully ladle the hot cordial into clean, hot jars, leaving appropriate headspace. Wipe the rims clean, apply lids, seals, and clamps and close securely.
Process in water bath: Place the jars in the already hot water bath canner and heat to 90°C. Once the temperature is reached, start timing and process for 30 minutes at 90°C. Then turn off the heat and let the jars rest in the water for 5–10 minutes.
Carefully remove the jars and place them on a towel or wooden surface. Let them cool undisturbed for 24 hours. After that time, remove the metal clamps and check the seals. Label the jars and store them in a cool, dark place.
Serving suggestion: Dilute to taste with water or sparkling water, or use in cocktails, spritzers, and desserts.
