Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with olive oil.
In a large bowl, whisk together the eggs, milk, garlic powder, onion powder, salt, black pepper, and nutmeg until well combined.
Stir in the chopped spinach, ricotta cheese, mozzarella cheese, Parmesan cheese, and sun-dried tomatoes if using. Mix until evenly combined.
Add the cubed French bread to the mixture, gently folding until the bread is well coated.
Pour the mixture into the prepared baking dish, spreading it out evenly.
Cover the dish with aluminum foil and refrigerate for at least 1 hour, or overnight for best results.
When ready to bake, remove the foil and bake for 35-40 minutes, or until the top is golden and the center is set.
Allow to cool for 5-10 minutes before slicing and serving.