01 - Grease a 9x13-inch baking dish thoroughly with butter or coat with cooking spray.
02 - Spread cubed bread evenly in the bottom of the prepared dish, creating a single, even layer.
03 - Heat olive oil in a large skillet over medium heat. Add the diced onion and sauté until softened, about 3–4 minutes. Add chopped spinach and continue cooking, stirring occasionally, until wilted.
04 - Distribute the sautéed spinach and onion mixture evenly over the bread. Sprinkle the shredded cheese evenly on top.
05 - In a medium mixing bowl, whisk together eggs, milk, garlic powder, salt, and black pepper until fully blended.
06 - Pour the egg mixture slowly and evenly over the layered ingredients in the baking dish. Gently press down on the bread to ensure it absorbs the custard.
07 - Cover the dish tightly with aluminum foil or plastic wrap and refrigerate for at least 6 hours, preferably overnight, to allow the flavors to meld and the bread to soak up the custard.
08 - Remove the dish from the refrigerator. Preheat the oven to 350°F (175°C).
09 - Uncover and bake for 45 to 55 minutes, or until the top is golden brown and the center is set. If the top browns too quickly, tent loosely with foil.
10 - Remove the strata from the oven and let it stand for 10 minutes before slicing and serving.