Serves 6 generously
Butter a 13 x 9-inch baking dish. Cut twenty 1-inch slices from baguette and arrange in one layer in baking dish. In a large bowl whisk together eggs, milk, nutmeg, vanilla, and 3/4 cup brown sugar and pour evenly over bread. Chill mixture, covered, until all liquid is absorbed by bread, at least 8 hours, and up to 1 day.
Preheat oven to 350°F.
In a shallow baking pan spread pecans evenly and toast in middle of oven until fragrant, about 8 minutes. Toss pecans in pan with 1 teaspoon butter and salt.
Increase temperature to 400°F.
Sprinkle pecans and blueberries evenly over bread mixture. Cut 1/2 stick butter into pieces and in a small saucepan heat with remaining 1/4 cup brown sugar, stirring, until butter is melted. Drizzle butter mixture over bread and bake mixture 20 minutes, or until any liquid from blueberries is bubbling.
Make syrup while French toast is baking:
In a small saucepan cook blueberries and maple syrup over moderate heat until berries have burst, about 3 minutes. Pour syrup through a sieve into a heatproof pitcher, pressing on solids, and stir in lemon juice. Syrup may be made 1 day ahead and chilled, covered. Reheat syrup before serving.
Serve French toast with syrup.