The night before, butter a 9×13″ baking dish
Cut challah into large cubes and arrange them in one layer in a baking dish
In a large bowl whisk together eggs, milk, nutmeg, vanilla, and the 3/4 cup of brown sugar. Pour evenly over bread.
Chill mixture, covered, until all liquid is absorbed by bread, at least 8 hours, and up to 1 day. If you have the time or remember, turning the chunks of bread over to get fully absorbed is helpful, but not 100% necessary.
The day of, preheat the oven to 400 degrees Fahrenheit.
Take half stick of butter and melt in a small saucepan with the remaining 1/4 cup of brown sugar until combined. PS when I’m being lazy, I have even done this in the microwave, and it turns out fine too.
Drizzle butter mixture over the bread and top with pecans and blueberries. Bake 30 minutes covered with foil, then remove the foil and bake another 10-15 minutes until golden brown.
While that bakes, make the blueberry syrup. In a small saucepan cook 1 cup blueberries and 1/2 cup maple syrup until the berries have burst (about 3 minutes)
Pour syrup through a sieve pressing on solids
Stir in 1 tbs fresh lemon juice