Thoroughly butter or grease a 9×13 casserole dish. Cut or rip each slice of sourdough into bite size pieces and layer in the dish.
In a large mixing bowl whisk eggs, milk, cream, sugar, vanilla, and cinnamon. Pour over the sourdough pieces and gently toss with a large spoon. Cover and refrigerate overnight.
The topping can be prepared the night before or in the morning while the oven preheats. To prepare, combine all topping ingredients and cut in the cold butter with a fork.
In the morning, preheat the oven 350 degrees, uncover the French toast, crumble the topping mix over the top, and bake for 45 minutes.
Serve plain or fancy it up with a dollop of whipped cream, maple syrup or jam.