Pre-heat oven to 350* F. Use 1 Tablespoon butter to grease 13" X 9" baking dish.
Heat 4 Tablespoons butter in a large skillet or sauté pan, and sauté yellow onions and mushrooms, about 3-4 of minutes until onions are translucent and mushrooms are soft. Add garlic and sauté another minute.
Add spinach, a handful at a time, and sauté. Cover skillet and let spinach wilt, about 5 minutes.
Let cool, drain excess liquid, and chop more finely.
In a separate bowl, whisk eggs, cottage cheese, cheddar cheese, green onions, and salt and pepper. Add cooked spinach and mushroom mixture.
Mix well and pour into baking dish.
Bake for 45-50 minutes or until top is golden brown and center is done.