Start by browning the sausage over medium heat with a little vegetable oil. Use a slotted spoon to transfer the cooked sausage to a medium-sized mixing bowl. Return the pan to medium-high and add the sliced mushrooms to the skillet. Sauté the mushrooms until they’ve given off their moisture and begin to brown. 

Add the baby spinach to the skillet and and sauté just until the spinach is wilted. Transfer the mushroom and spinach mix to the bowl with the sausage and toss to mix together. Set aside to cool slightly. 

Meanwhile, whisk together the 6 eggs, milk and Dijon mustard. Season the egg mixture with 2 teaspoons of kosher salt and black pepper.

Add about 5 ounces (a little more than half) of the grated Fontina cheese to the sausage, mushroom, spinach mixture and toss to combine. 

Pour the egg mixture into the sausage mixture and stir to combine. 

Lightly grease a 12×9 casserole dish. Then tear four, day-old croissants into the greased dish. 

Pour the egg, sausage, mushroom, spinach, cheese mixture over the torn croissants. Use a spatula to spread the mixture evenly in the dish. 

Top with the remaining grated Fontina cheese. Then you can top the cheese with sliced scallions now, or you can top with sliced scallions after it comes out of the oven. 

Cover the casserole and refrigerate for at least 8 hours or up to overnight.

When you’re ready to bake, preheat the oven to 375º and let the strata come up to room temperature while the oven preheats. Place the casserole dish on a sheet pan and bake the strata for about 1 hour, covering if needed to prevent over-browning. 

The strata is ready when the center is slightly firm to the touch and the top is golden brown. Let the strata cool for about 10 minutes before slicing and serving.