Fruit filling from K8
I make one by cooking strawberries, blueberries, raspberries and blackberries in a heavy sauce pan with a peeled diced granny smith apple and one third cup of sugar and a squirt of lemon juice and simmer gently stirring often for about 30-45 mins
But you can use any single berry or any combination. And they can be fresh, frozen or a combination
And the pectin in the apple is what sets it--it's really cool--I feel at my pastry chefinest when I make this
Optional--if you cook it shorter or want insurance you can add in a little mixture of 1 tablespoon of cornstarch and 2 tablespoons of water poured into bubbling fruit stuff stirring constantly for another two minutes.
Cool, puree in food processor and then strain out the seeds. Add almond extract if you want. Be sure to securely seal the edges of the filling with buttercream.
Then I often mix in some lemon cream (it's super zipped up lemon curd)
The fresh bursting taste is well worth the little bit extra effort. Enough for a quarter sheet.