Fresh berries, dense white bread, and light-brown sugar are the only ingredients in this delicious summer pudding.
Cut a piece of cardboard to fit snugly inside a pullman loaf pan. Set aside. Place a half pound each of blueberries and blackberries in a large skillet; sprinkle with 1/2 cup brown sugar. Add 1/2 cup water; place skillet over medium-low heat. Cook, stirring occasionally, until sugar is dissolved and berries soften and release juices, about 10 minutes.
Remove from heat, and stir in the reserved half pound each of blueberries and blackberries. Transfer berries and their juice to a bowl, and set aside until cooled. Repeat this procedure with the raspberries and strawberries, using same skillet, or simultaneously with another skillet.
Line the inside of a 13-by-4-by-4-inch pullman loaf pan with plastic wrap, making sure it overhangs the edges by about 4 inches on each side. Cut the bread into 3/8-inch-thick slices. Line the bottom and sides of the pan with the bread slices, trimming if needed. Slices must be cut to fit snugly, with no gaps between them and no overlap.
Using a slotted spoon, place the blueberry mixture in the bottom of the prepared terrine in an even layer, leaving the juices in the bowl. Press down lightly to make a flat layer. With the slotted spoon, spoon in the raspberry mixture over the layer of blueberry mixture. Cover the terrine evenly with a layer of bread. Strain and combine the reserved juices through a fine sieve into a glass measuring cup. Discard the solids, and pour the juice evenly over the terrine, until it is almost full. Don't overfill, or juice will spill out when pressed down. Reserve any excess juice for unmolding. Press down gently with your hands to compress terrine; fold excess plastic wrap over to cover. Place reserved cardboard over terrine; weigh down with soup cans or jars of jam.
Chill overnight weighted, so the bread is pressed into the berries and their juice. Unmold onto a serving platter. Use reserved berry juices to brush over pale spots if needed. Cut crosswise into 3/4-inch slices; serve with more berries and cream.
Use fresh white pullman bread, not presliced packaged bread, which can make the pudding gummy.