Wash the strawberries and remove the stems and leaves. Dice them and gather the strawberries in a bowl. Slice thinly the lemons and add them to the strawberries. Cover the fruit with sugar, stir and marinate for at least 2 hours, until the sugar has melted into a syrup.
Pour the fruit and the syrup in a thick-bottomed pan and bring to a boil over low heat. Simmer gently for about 45 minutes stirring often. Check if it is ready dropping some jam on a cold plate: if it doesn't slide over when you tilt the plate, it's ready.
Pour the jam into 3 sterilized jars and close tight. Put the jars in a deep pot, cover with water and bring to the boil. Simmer for 20 minutes, then remove from the heat. Let the jars cool completely in the pot. The jam will keep for several months in a cool, dry, dark place.