Preheat oven to 140 degrees (or as low as your oven will go). Line a rimmed half-size sheet pan with parchment paper or a silicone mat. You can use a larger pan, but your mixture will not cover the full sheet pan.
In a food processor or blender, puree fruit with sweetener and lemon juice until perfectly smooth.
Pour pureed fruit onto the prepared pan and use an offset spatula to evenly spread the fruit out so that it is about ¼ inch thickness. Be sure it is evenly spread for even baking.
Place in oven and bake for 4-6 hours, until set and no longer sticky to the touch. ***See note
Remove from the oven and allow to cool fully. If you use parchment paper, you can leave the fruit roll on the parchment paper, if you baked your fruit leather on a silicone baking mat, peel it off before cutting it into strips.
Cut the cooled fruit leather into 1-inch strips and roll up in parchment paper or wax paper and secure with tape or a sticker to make it cute if desired.
Be sure to spread your fruit mixture thin, but without any holes--so an even layer about ¼ inch thick.
I used 2 cups of strawberries and 1 cup of blackberries for this recipe.
You can use frozen fruit. Just defrost first in a strainer over a bowl in the refrigerator to remove excess water. Drain well and pat dry before proceeding with the recipe.
It can take up to 8 hours to cook these fruit roll-ups if your fruit has high moisture content and your oven is set at 150 degrees. I have only ever had to bake my fruit roll-ups for 5 hours. But you may need longer. Check every 30 minutes and remove from oven when no longer sticky to the touch.
Keep homemade fruit-roll ups in a tightly sealed container at room temperature for up to 2 weeks.