Blueberry Sauce: Combine blueberries, granulated sugar, lemon juice and cornstarch in a medium saucepan. Bring to a boil, then lower heat to a simmer and cook 10-11 minutes until some of the blueberries burst and the mixture has slightly thickened. It will continue to thicken a bit as it cools.
Bread Pudding: Preheat oven to 350 degrees. Spray a 6 cup baking dish with nonstick cooking spray. Set aside.
Place the croissant pieces in the dish, sprinkling 1/2 cup of the blueberries in between.
In a large bowl, whisk together half and half, eggs, sugar, lemon zest, salt and vanilla extract. Pour over the croissant pieces. Use a spatula to press down on the croissants to make sure all pieces have been moistened. Top with remaining 1/2 cup blueberries. Let mixture soak for 25 minutes.
Top with almond slices and sprinkle with turbinado sugar. Bake for 50-60 minutes or until a knife inserted in center of pudding comes out clean. Remove from oven and let stand for 15 minutes before serving with warm blueberry sauce on the side.