In a large bowl, combine the cooked sausage, cubed bread, cheddar, spinach, and tomatoes, and toss together well until evenly distributed. Pour this mixture into the buttered casserole dish.
In another bowl, whisk to combine the eggs, until lightly blended. Add the heavy cream and milk, and add a big pinch of salt and pepper. Whisk until smooth, then pour it all over the bread cube mixture in the casserole dish. (You will notice that the liquid does not completely cover all of the bread, and that’s fine. By the time you wake up in the morning, the liquid should be absorbed.)
Cover the casserole dish with plastic wrap or foil and store in the fridge overnight, or for up to 18 hours.
When you wake up in the morning, preheat the oven to 350 degrees F and bake the casserole for 1 hour until the custard has set and the top is golden brown. Serve and enjoy!