Combine butter, half-and-half, sugar, eggs, vanilla and nutmeg in a bowl. Beat until smooth. Stir in frozenmixed berries.
Place bread cubes in a large bowl. Pour fruit mixture over bread cubes. Toss until bread is well moistened.
Spoon into prepared pan. Cover pan securely with foil. Cut a piece of foil 2 feet by 1 foot and double it twice lengthwise to create a strip for moving the pan into and out of the pressure cooker.
In a 5-quart or larger pressure cooker, add 1 1/2 cups water and insert trivet. Lower the pan onto the trivet with the prepared foil harness. Close lid and bring pressure to the second red ring (15 pounds pressure) over high heat. Adjust heat to stabilize at the second red ring.
Cook 15 minutes. Remove from heat and allow to release pressure naturally (Natural Release Method).
Remove lid and lift the pan from cooker. Place on wire rack to cool before serving or servewarm if preferred.