In a large mixing bowl add the half and half, eggs, maple syrup, and vanilla. Chop the croissants, and arrange them in a 13 x 9 baking dish. Pour the egg mixture over the croissants, and sprinkle pecan pieces on top.
To make the crumble: Add the butter, brown sugar, salt, and oats in a bowl. With fingers or a pastry cutter, rub the mixture together until it resembles a coarse crumble. Sprinkle over the croissants. For the best results, cover and refrigerate overnight in order for the croissants to soak up some of the liquid.
Preheat the oven to 350 degrees F. Bake the croissant bread pudding, covered with tin foil, for 20 minutes. Remove the foil, and continue to bake for an additional 30 minutes or until crisp and fragrant.
Store in the fridge for up to 3 days or in the freezer for up to 3 months.
