Add the raspberries, blackberries, blueberries, and sugar to a medium saucepan over medium heat. Cook for 8-10 minutes, stirring occasionally, until the berries have softened but are still mostly intact and the sugar just dissolves. Remove the pan from the heat.
2. Step
Strain the berries through a fine-mesh sieve set over a liquid measuring cup. Add water as needed so you have 1½ cups (360 ml) of liquid total. Pour the liquid into a shallow dish. Reserve the fruit and the liquid.
3. Step
Trim the crusts off the bread. Cut the bread in half down the middle.
4. Step
Line a 2-quart bowl with enough plastic wrap to hang over the edges at least 4 inches (10 cm) all around.
5. Step
Dip a half slice of bread at a time in the reserved berry juice, then line the sides of the wrapped bowl, overlapping slightly. Some slices can hang over the side of the bowl. Line the center of the bowl once the sides are covered.
6. Step
Spoon the reserved berries into the center of the lined bowl. Fold the bread over the berries, then dip the remaining bread in the berry juice and cover the center, overlapping slightly. Pour any remaining juice over the pudding.
7. Step
Cover the pudding with the plastic wrap. Place a small plate on top and weigh down with a can. Refrigerate for at least 6 hours, up to overnight.
8. Step
When ready to serve, remove the plate and the can and peel back the plastic. Place a serving platter over the bowl and quickly flip both the bowl and platter over to invert the pudding. Remove the bowl and plastic wrap.
9. Step
Top the pudding with the whipped cream, more berries, and fresh mint leaves. Slice and serve.
