1/2 cup pure maple syrup, plus additional for serving
6 cups 1-inch cubes day-old country-style white bread with crusts (about 12 ounces or 3/4 loaf)
1 pint fresh raspberries
6 large eggs
4 cups whole milk
1 teaspoon finely grated lemon zest
1/4 teaspoon salt
Arrange rack in center of the oven and preheat to 350 F.
Lightly butter 9- by 13-inch glass baking dish or 14-inch oval gratin dish. Pour in maple syrup; scatter bread cubes and raspberries in dish.
In large bowl, whisk together eggs, milk, lemon zest, and salt; pour over bread cubes.
Bake strata until puffed and golden brown, about 45 minutes. Transfer to rack to cool 5 minutes, then serve warm with additional maple syrup.