Mix together the brown sugar, melted butter, and 1/2 of the maple syrup. Spread mixture into a 12" round (or 9"x13") baking dish lightly coated with cooking spray. Top with 3⁄4 cup of the pecans. Place french bread slices evenly on top.
Whisk together next six ingredients. Pour over bread slices. Cover and chill for 8 hours.
Preheat the oven to 350°F. Remove baking dish from refrigerator and let it stand at room temperature for 10 minutes. Bake, uncovered, until golden brown and set, about 45 minutes.
Sprinkle casserole with powdered sugar and remaining pecans. Serve with remaining syrup.