Spray a 13X9 casserole dish with nonstick cooking spray. Layer bread cubes in the bottom of the dish.
In a skillet over medium heat, cook the turkey sausage, onion, and garlic together. Once it has reached 165 degrees F add in the drained spinach. Let sausage cool, and then layer on top of the bread cubes.
In a medium bowl whisk milk, eggs (whites and whole), salt, and pepper until combined. Pour over the sausage mixture in the casserole dish. Sprinkle cheese over the top of the strata and cover with foil.
Place in the refrigerator overnight but no more than 24 hours.
Preheat over to 350 degrees F. Remove the strata from the refrigerator, place in the oven covered with foil for 30 minutes.
Remove foil and bake for additional 15 minutes until the center is set.