Preheat your oven to 375°F (190°C) and grease a 9 x 13-inch baking dish with salted butter.

Sauté the breakfast sausage in a skillet over medium heat until browned and fully cooked, about 5–7 minutes. Set aside to cool.

In the same skillet, sauté the mushrooms until golden for about 4–5 minutes, then add spinach until just wilted, about 1–2 minutes.

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

Fold in the cooked sausage, sautéed mushrooms, wilted spinach, and 1.5 cups of shredded cheese until evenly mixed.

Pour the egg mixture into the greased baking dish and spread evenly across the bottom.

In a separate bowl, toss the bread cubes with olive oil, kosher salt, and black pepper, then layer over the egg mixture.

Sprinkle remaining shredded cheddar cheese on top and bake in the oven for 35–40 minutes, until the center is set and croutons are golden brown.

Let cool for 5 minutes before slicing and serving warm to enjoy.