1 (10-ounce) package chopped frozen spinach, defrosted and well drained
Butter a large casserole dish. Heat 2 tablespoons butter in a medium heavy skillet over medium heat. Add the onions and cook, stirring occasionally, until golden brown and caramelized, about 20 minutes. Transfer onions to a bowl and wipe out skillet.
Add the chopped bacon to the skillet and cook over medium-low heat, stirring occasionally, until golden brown and crisp, about 10 minutes. Remove from heat and, using a slotted spoon, transfer bacon to a paper towel–lined plate. Wipe out skillet.
Add the sausage to the skillet and cook over medium heat until browned all over (8–10 minutes for frozen sausages; 5–6 minutes for thawed). Remove from heat and slice sausages into bite-sized pieces.
Crack the eggs into a large mixing bowl and add the milk, dry mustard, salt and pepper. Whisk together until well combined. Stir in the bread, bacon, sausage, caramelized onions, cheese, and spinach. Pour mixture into the prepared casserole dish, pressing down on the bread to submerge it.
Cover with plastic wrap and refrigerate for at least 4 hours or overnight, to allow the bread to soak up the egg mixture.
Preheat the oven to 350°F. Discard plastic wrap and bake strata until top is golden brown and mixture is firm to the touch, about 1 hour. Serve warm.