Grease 9×13 baking dish with non-stick spray.
Lay bread in the dish, top with butter and bake 8 mins to toast. (Next time I may toast both sides!)
Cool completely.
Scatter turkey in dish around bread.
Mix the custard by whisking eggs, milk, dijon, sage, s+p, spinach and 1 1/2 c. of the cheese. Reserve rest for the top.
Pour this over the bread, sprinkle on more cheese & prosciutto and cover in fridge overnight.
Bake at 350 for 45-55 mins.
Cool a few mins and serve.