Trim the stems off the figs. Prick each one 4 to 5 times with a fork or skewer so the syrup can soak in.

Put the figs in a large bowl of cold water and soak for 4 hours, or overnight in the fridge. This keeps the skins from splitting later.

Drain the figs. Bring a pot of water to a boil and blanch them for 3 to 4 minutes, just until they start to soften. Drain again.

In a 4-quart / 3.8L heavy-bottomed pot, combine the sugar, 500 ml water, lemon slices, and cloves. Bring to a boil, stirring until the sugar fully dissolves.

Add the blanched figs to the syrup. Return to a boil, then lower the heat so it's just simmering.

Simmer uncovered for 45 to 50 minutes, spooning the syrup over the figs every 10 minutes or so, until the figs turn deep amber and translucent and the syrup coats the back of a spoon (about 106 C / 223 F).

In the last 5 minutes, stir in the lemon juice and the mastic, if using.

Remove the pot from the heat and let it sit for 15 minutes to cool slightly.

Ladle the figs and syrup into sterilized jars while still warm, leaving a small gap at the top. Seal the lids tightly.

Let the jars cool fully on the counter before storing them in a cool, dark pantry.