In a small skillet over medium heat, melt the butter, and saute the diced onions for about 8 to 10 minutes or until soft. Remove from the heat and allow to cool.
2 tbsp Unsalted butter, 1 cup Onion
Butter a 9×13 baking dish. Set aside.
In a large bowl combine the torn bread pieces or cut bread cubes, diced ham, and sauteed onions. Toss lightly just to help cool down the onions.
8 cups Italian bread, 2 cups Smoked ham
Add the 1 1/2 cups of the shredded Gruyere and grated Romano. Toss to combine. You can add all the cheese to the mixture at this time, instead of reserving 1/2 cup to sprinkle over the top during baking.
2 cups Gruyere cheese, 1/4 cup Pecorino Romano
In another bowl whisk together the eggs, milk, heavy cream, chives, thyme, dry mustard, kosher salt, and pepper until combined.
8 large Eggs, 1 1/2 cups Whole milk, 1/2 cup Heavy cream, 2 tbsp Fresh chives, 1/2 tsp Dried thyme, 1/2 tsp Ground mustard, 1 1/2 tsp Kosher salt, 1/4 tsp Black pepper
Pour the egg mixture over the bread mixture and gently fold all the ingredients together.
Pour the mixture into the prepared baking dish, smooth the top, and wrap tightly in plastic wrap. Refrigerate overnight or at least 6 hours.
Preheat the oven to 350 degrees F. Remove the plastic wrap and cover the baking dish with aluminum foil sprayed lightly with nonstick cooking spray.
Remove the strata from the refrigerator for at least 30 minutes before placing it in the oven.
Bake for 30 minutes covered, remove the foil and bake an additional 30 minutes, or until the strata is lightly browned, puffed up, and a knife inserted into the center comes out clean.
Let cool for 15 minutes before serving.