In a heavy-bottomed saucepan combine raspberries and sugar and let rest for about 10 minutes so the fruit releases its juices.

Place the pan over medium heat and stir gently until the sugar fully dissolves and the mixture starts to bubble.

Increase heat to medium-high and cook, stirring frequently and skimming off any foam, until the mixture reaches 104 °C on a candy thermometer or until a small spoonful chilled on a plate wrinkles when nudged, about 15–20 minutes.

Stir in lemon juice, zest if using, and salt, then simmer for an additional minute to integrate the flavors.

Remove from heat and immediately ladle the hot jam into sterilized jars, leaving about 6 mm of headspace. Wipe the rims clean and seal with lids.

Allow jars to cool undisturbed at room temperature. Once cooled, refrigerate for up to three weeks or process in a boiling-water bath for 10 minutes for shelf-stable storage.