Heat olive oil over medium heat.
Add onion and cook until they begin to soften, 4-5 minutes.
Add mushrooms and cook another 5-7 minutes, until mushrooms begin to release moisture.
Add spinach and toss until wilted. Remove from heat.
Add mushroom mixture to bread, tossing to combine.
Place in 9x13" baking dish and top with cheese.
In medium bowl, whisk together eggs, then add milk, dijon mustard, garlic powder, salt, and pepper.
Pour egg mixture over cheese.
Refrigerate mixture for 2 hours, or up to overnight, covered.
Preheat oven to 325 degrees F.
Bake, uncovered, for 55-65 minutes, until golden brown and cooked through.
Serve and enjoy!