Tear or cut the brioche loaf into bite-sized pieces, and spread evenly in the dish.
In a large mixing bowl, mix together the eggs, eggnog, sugar, nutmeg, and vanilla. Pour the mixture over the bread, ensuring all pieces are coated.
Cover the baking dish, and place in the refrigerator for at least 4 hours, preferably overnight.
When ready to bake, remove from the refrigerator, and uncover.
Preheat the oven to 350 F.
In a medium-sized mixing bowl, mix together the pecans, brown sugar, flour, butter, and cinnamon. Sprinkle the mixture over the top of the brioche in the baking pan.
Bake for 45 to 50 minutes, or until the French toast custard has set and the top is golden.