Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish with salted butter.

In a large skillet over medium heat, cook the breakfast sausage until browned, about 8-10 minutes.

In the same skillet, melt 2 tablespoons of salted butter and sauté the sliced mushrooms for 5-6 minutes.

Add baby spinach and cook for an additional 3-4 minutes until wilted.

In a mixing bowl, whisk together the eggs, whole milk, Dijon mustard, black pepper, and kosher salt.

Combine cooked sausage, sautéed mushrooms, and spinach into the egg mixture, folding in 1.5 cups of cheddar cheese.

Toss fresh bread cubes with olive oil, salt, and pepper, ensuring they're well coated.

Pour the egg mixture into the baking dish, distribute the bread cubes on top, and sprinkle remaining cheddar.

Bake for 35-40 minutes until set and the top is golden brown.

Let cool for 5 minutes, garnish with parsley, and serve warm.