Line an 8” square baking dish with parchment - allow the excess to hang over the sides for easy removal later.
Combine the applesauce and berries in the food processor. You can use a blender too.
Blend until the mixture is smooth and you have a puree.
Transfer the mixture to a glass bowl or the top of a double boiler. If using a glass bowl, nest it on top of a small saucepan filled with water. Fill the double boiler halfway with water.
Sprinkle the gelatin into the puree.
Heat over medium heat, whisking frequently until the gelatin has dissolved. The mixture will become thin and it will be warm. Remove before it feels hot.
Allow the mixture to cool at room temperature for 10 minutes. You don’t want to beat the mixture while it is too warm.
Transfer to a stand mixer with the whisk attachment.
Beat on high for 15 minutes. The mixture will increase in volume by 4x. It should have a thick, mousse-like texture and hold a soft peak.
Spread the mixture into the parchment-lined pan.
Cover and refrigerate for at least 1 hour. I like to make mine ahead of time and refrigerate overnight.
Keep refrigerated until serving.
Top with additional berries, a dusting of confectioner’s sugar, frosting, whipped cream or whatever else you desire.