Grease a 9×13 baking dish with butter and set aside.
Shingle the slices of bread into the prepared baking dish. Sprinkle 1 ½ cups of blueberries over the slices of bread, allowing them to fall into the cracks.
1 loaf challah bread, 2 cups blueberries (fresh or frozen)
To make the custard, crack eggs into a large bowl. Whisk until smooth. Add the milk, heavy cream, salt, zest, lemon juice, sugar, vanilla, and cinnamon. Whisk to combine and pour over the prepared bread, ensuring to moisten each slice. For overnight French toast, cover tightly with plastic wrap and chill for up to 8 hours.
7 large eggs, 1 ¾ cups whole milk, 1 cup heavy cream, ½ tsp salt, zest from one lemon, 2 tbsp lemon juice, 2 tsp vanilla extract, 1 1/2 tsp cinnamon, ½ cup sugar
If wanting to bake the same day, allow the bread to soak for 30-60 minutes. When ready to bake, preheat the oven to 350°F. Top the French toast with remaining blueberries and chopped pecans. Bake until puffed and golden brown, 45-55 minutes.
1 cup toasted and chopped pecans
Remove from the oven and cool for 10 minutes. Serve with powdered sugar and maple syrup.