Lightly spray an 11x 7” baking dish.
Heat the oil in a large, nonstick skillet over medium heat. Add the sausage and cook, stirring often, until the meat is cooked through, about 5 minutes.
Line the baking dish with overlapping slices of bread.
Sprinkle the zucchini evenly over the bread.
Spoon the sausage over the zucchini and sprinkle evenly with the cheese.
In a large bowl, stir the eggs with a fork until broken up.
Add the mustard then the milk into the bowl with the eggs and stir until well-blended.
Pour the egg mixture over the bread mixture in the baking dish.
Cover and refrigerate for at least 4 hours or overnight.
To bake, preheat the oven to 350 degrees.
Uncover the casserole and bake about 45 minutes or until the top is puffy and browned.
Allow to cool for 5 - 10 mins before serving.
Enjoy!