Hello GardenPreserver,
Fresh garden fruit is wonderful for baking, but it presents unique challenges because homegrown produce is typically juicier and varies in pectin content compared to commercial fruit. When the fruit softens and loses its cell structure during baking, the internal volume decreases significantly, creating that hollow or sunken center you are experiencing.
To achieve a stable, beautifully domed pie, I recommend a few methodical adjustments:
Mound your fruit filling in the center. When filling the pie shell, arrange the fruit into a gentle dome shape, piling it higher in the middle than at the edges. As the fruit inevitably softens and shrinks under heat, this extra height compensates for the volume loss, settling into a flat or gently rounded top rather than a sunken cavity.
Pre-gelatinize your filling juices. Toss your prepared fruit with sugar to draw out excess liquid, then simmer just the drawn juices in a small saucepan with your starch (such as instant tapioca or cornstarch) until thickened and clear before gently folding it back over the fruit. This locks in the moisture without overcooking the delicate fresh fruit.
Bake on a preheated baking stone or heavy sheet pan on the lowest rack. Direct, intense bottom heat helps set the lower crust quickly and activates the thickeners in the center of the pie much faster, ensuring the filling gelatinizes properly before the top crust completely hardens.
I hope these techniques help you get the consistent, beautiful results your garden harvest deserves.