Heat a large skillet over medium heat and add the butter. Once the butter is melted add the mushrooms. Saute for 3-4 minutes, stirring occasionally. Once the mushrooms begin browning, add the spinach and saute briefly until it wilts. Remove from the heat and place in a large bowl.

Add the cooked, crumbled sausage and diced bread to the mushrooms and spinach.

In another bowl whisk together eggs, milk, thyme, seasonings, and dijon. Whisk in the cheese. Pour over the bread mixture and toss until combined. Cover and set in the fridge for 30 minutes to an hour.

Preheat the oven to 350 degrees fahrenheit and spray a 9x13 or 4 quart baking dish with pan spray. Add the chilled strata mixture to the pan. Bake for 45-50 minutes, until the top is golden brown and the center is set and no longer jiggly.

Let the strata sit for 5-10 minutes before serving.