In a large skillet, heat the olive oil over medium-high heat. Add the onion and cook until tender, about 5 minutes. Add the sausage and cook through until no longer pink, about 5 more minutes. Drain off any fat from the pan. Squeeze the spinach dry and add to the pan with the onions and sausage, add a big pinch of salt and pepper and cook for 2 minutes. Remove from heat.

Layer the bread cubes on the bottom of a 9 by 12 inch (or similar sized) baking dish. Add the sausage and spinach mixture then top with the cheese.

In a mixing bowl, whisk the eggs with the milk, 1 teaspoon salt, ½ teaspoon pepper, and the mustard powder. Pour the egg mixture into the baking dish. Cover and refrigerate for about 8 hours or overnight.

Preheat oven to 350 degrees. Remove the strata from the refrigerator and let sit at room temperature while the oven preheats. Bake the strata for 50 – 60 minutes or until everything is set. Let cool a few minutes and serve.