Savoury Overnight French Toast Bake

Half (approx. 250 g) of a large loaf of day-old French bread, cubed 3 eggs2 tsp. smooth Dijon mustard1/4 tsp. onion powder2 tbsp. chives, chopped1 cup milk3 oz. (a little more than 1/3 cup) of Gruyere*, shreddedSalt and pepper