1 Assemble the dry ingredients: Butter a 9×13 (or other 3-quart) baking dish. Spread half of the bread cubes in an even layer in the prepared pan. Top with 2/3 of the blueberries, followed by the remaining bread cubes and blueberries.
If using frozen blueberries, add them to the casserole while still frozen (do not thaw). This helps prevent the berries from coloring the casserole too much.
2 Whisk the liquids: Beat the eggs, milk, cream, vanilla, 6 tablespoons of the sugar, lemon zest, and the salt together. Pour the mixture over the bread cubes, pressing down gently (so you don’t burst the berries) to make sure the egg mixture is absorbed.
3 Soak at least 30 minutes, or overnight: If soaking overnight, cover the pan with plastic wrap and refrigerate overnight.
4 Preheat the oven to 350°F about 15 minutes before you plan to bake the casserole.
5 Bake the casserole: Just before baking, sprinkle the top of the casserole with the remaining sugar and dot with the diced butter. Bake for 45 minutes, until slightly puffed and golden brown. (Casserole that chilled overnight might take a few minutes longer.)
6 Serve: Let the casserole cool for about 10 minutes before cutting. Serve with a dusting of powdered sugar over top or a drizzle of maple syrup.
Leftovers will keep, refrigerated, for about a week and can be reheated in the microwave.