Preheat oven to 375° Fahrenheit. Lightly grease baking dish, then layer spinach leaves, white parts of scallions, and chopped parsley in bottom of baking dish. Set dish aside.
In large skillet over medium heat, cook breakfast sausage until browned and crumbled. Stir often while cooking and break sausage into small pieces with spatula or wooden spoon.
While sausage cooks, whisk together eggs, heavy cream, garlic powder, salt, and pepper in large mixing bowl. Once ingredients are fully combined, gently fold in shredded cheddar cheese.
Layer cooked sausage on top of spinach leaves in baking dish. Pour egg mixture over sausage, making sure to distribute mixture as evenly as possible across baking dish.
Place baking dish in preheated oven and bake 20 to 25 minutes or until casserole is firm and eggs are cooked through. Remove from oven and slice into 12 squares. Garnish with green parts of scallions and additional shredded cheese if desired, then serve warm.