Toss berries with sugar and lemon zest. Set aside 10 minutes.
Grease a 9x13 inch baking dish generously.
Arrange bread slices in overlapping rows in the dish.
Scatter berries and all their juices over and between the bread. Tuck berries down between slices.
Whisk eggs, milk, cream, sugar, vanilla, cinnamon, nutmeg, and salt until smooth and sugar is dissolved.
Pour custard slowly and evenly over the entire dish, including between slices.
Press bread down firmly to absorb the custard.
Cover tightly and refrigerate overnight or at least 8 hours.
Remove from fridge 20–30 minutes before baking.
Preheat oven to 180°C (350°F).
Scatter butter cubes over the top. Mix brown sugar and cinnamon and sprinkle evenly over the surface.
Bake uncovered 40–45 minutes until deep golden on top and set at the edges with only a faint wobble in the center.
Rest 10 minutes before serving.
Dust with powdered sugar. Scatter fresh berries. Serve with maple syrup and whipped cream.