In a small saucepan, combine fruit, water or tea, and sweetener if using. Heat over medium-low until fruit is soft and mixture is warm.
Use an immersion blender or transfer to a blender to puree until smooth.
Return to the saucepan (off the heat) and whisk in lemon juice. Slowly sprinkle in the gelatin while whisking continuously to avoid clumping.
Once smooth and well combined, return to low heat and stir gently for 1–2 minutes until fully dissolved (do not boil).
Pour mixture into silicone molds or a parchment-lined dish.
Refrigerate for 1–2 hours until set.
Pop out of molds or cut into squares. Store in an airtight container in the fridge for up to 7 days.