Wash and peel the guavas if desired. Chop them into small chunks and discard seeds if you prefer a smoother jam.
Place chopped guavas in a saucepan with one cup of water. Bring to a boil, then lower the heat and simmer for 15-20 minutes until softened.
Blend the mixture using either an immersion blender or a standard blender. Strain through a fine mesh sieve if you want a seedless, smooth consistency.
Return the strained puree to the pot. Stir in the sugar and lemon juice. Simmer over medium-low heat until the jam thickens (20-30 minutes).
Use a kitchen thermometer (220°F is the setting point for jam) or the cold plate method to check the setting point.
Pour the hot jam into sterilized jars. Seal and turn upside down for 5 minutes to ensure vacuum sealing. Allow it to cool to room temperature before placing it in the fridge or storing it.
