Drain the thawed spinach, using your hands to squeeze as much water from it as possible. Once the spinach is drained finely chop it.
Heat a large skillet over medium heat and add butter and onions. Saute until onions soften.
Place spinach in the pan and add 1/2 tsp of salt and 1/4 tsp of pepper. Once combined remove from heat.
Mix the two cheeses together in a bowl.
Grease a 3 quart gratin dish with butter and spread one third of the bread across the bottom of the dish.
Next add a layer of spinach using 1/3 of the spinach mixture.
Layer 1/3 of the ham on top of the spinach.
Sprinkle 1/3 of the cheese over the ham.
Repeat laying two more times.
Whisk together eggs, milk, mustard, and 1/2 teaspoon of salt and 1/4 teaspoon of pepper.
Pour the egg mixture over the strata then cover it with plastic wrap and place in the refrigerator overnight (at least 8 hours, up to 1 day)
When ready to bake the strata preheat the oven to 350°F.
Bake uncovered for 45-55 minutes or until puffed, golden brown, and cooked through.
Let stand for 15-20 minutes before serving.