Prepare the Bread
Cut the brioche or challah into 1-inch cubes. Spread evenly in a buttered baking dish. Slightly stale bread works best because it absorbs more custard without becoming mushy.Make the Custard Mixture
In a large mixing bowl, whisk together eggs, half-and-half, milk, sugar, vanilla extract, orange zest, and salt until fully combined and smooth.Assemble the Casserole
Pour the custard mixture evenly over the bread cubes. Gently press the bread down so every piece absorbs the liquid.Refrigerate Overnight
Cover the dish tightly and refrigerate for at least 6 hours or overnight. This step is essential for deep flavor and perfect texture.Prepare Streusel Topping
In a bowl, mix brown sugar, flour, and cinnamon. Add cold butter cubes and use your fingers or a pastry cutter to create a crumbly texture. Stir in pecans if using.Bake the Casserole
Preheat oven to 175°C (350°F). Remove casserole from fridge and sprinkle streusel evenly on top. Bake uncovered for 45–55 minutes until golden brown and set in the center.Rest and Serve
Allow the casserole to rest for 10 minutes before serving. Top with maple syrup, powdered sugar, or fresh berries.