In a large skillet, heat the olive oil over medium heat. Add the sliced cremini mushrooms and cook until they are golden brown and tender, about 5-7 minutes.
Stir in the packed spinach leaves and minced garlic into the skillet. Season with Kosher salt and black pepper to taste. Sauté until the spinach has wilted, about 2-3 minutes. Remove from heat and let cool slightly.
In a large mixing bowl, crack the large eggs and whisk them together with the milk. Fold in the sautéed vegetable mixture, along with the sliced green onions and half of the goat cheese.
Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking dish with olive oil or cooking spray. Pour the egg and vegetable mixture into the baking dish, spreading it evenly.
Crumble the remaining goat cheese over the top of the casserole. Bake in the preheated oven for 35-40 minutes, or until the eggs are set and the top is lightly golden.
Once baked, remove the casserole from the oven and let it cool for a few minutes. Garnish with freshly chopped chives, if desired. Cut into squares and serve warm. Enjoy your delightful Spinach Mushroom Breakfast Casserole!
