Slice the baguette into 1-inch cubes, to make approximately 10 cups of loosely packed cubes. Add to a 9x13 pan. The bread should fill the pan, but not to overflowing.

In a medium bowl, add the milk, eggs, maple syrup, cinnamon, nutmeg, and salt. Whisk until frothy and well combined. Pour the egg mixture over the bread, and toss to coat.

Sprinkle the blueberries and sliced strawberries over the top of the mixture. Cover with foil and leave refrigerated for at least 8 hours, or overnight.

Preheat oven to 350 degrees Fahrenheit.

In a small bowl, combine the chilled butter pieces, brown sugar and chopped walnuts. Using your fingers, combine the ingredients using a pinching technique. Break up each of the butter pieces between your fingers, simultaneously combining it with the sugar and walnuts. The mixture will have a somewhat pebbly texture once you're done.

Remove the chilled bread mixture from the refrigerator and sprinkle the walnut mixture over the top of the bread. Cover again with foil and bake for about 30 minutes. Remove the foil and bake for another 10-15 minutes, until golden.

Remove from oven, and allow the casserole to rest for about 5 minutes before serving. Top with more berries and additional maple syrup if desired.