In a medium saucepan, combine the chopped guavas and water. Bring to a simmer over medium heat. Cook for 10–15 minutes, stirring occasionally, until the fruit is soft and beginning to break down.

Use an immersion blender on low speed to puree the mixture without crushing the seeds too much (which can cause a gritty texture). Alternatively, transfer to a regular blender and blend on the gentlest setting.

Pour the puree through a fine mesh sieve into a bowl, pressing with the back of a spoon to extract the liquid and pulp while leaving seeds and coarse bits behind. For a chunkier, rustic jam, strain lightly or skip straining altogether.

Return the strained puree to the saucepan. Stir in the sugar, lemon juice, and lemon zest. Heat over medium-low, stirring frequently, until the sugar is dissolved.

Continue cooking for 10–15 minutes, stirring often, until the jam thickens. You should be able to see the bottom of the pan briefly when you stir. To test, spoon a small amount onto a chilled plate — if it wrinkles when pushed with your finger, it’s ready.

Carefully ladle the hot jam into clean jars. Let cool at room temperature, then refrigerate. The jam will continue to thicken as it chills.