Bring 450 ml (approximately 15 ounces) of water and the sugar to boil in a pot. Cook for 5 minutes, remove from the heat, and add the lemon juice. Wipe the raspberries, add to the syrup, then mash with a potato masher and set aside for 3 hours.
Fill the syrup into clean bottles, then seal. Place one tablespoon of syrup into a glass, then fill with mineral water and serve.