Spray a 9x13in baking dish with a nonstick spray. Set aside.
Whisk the 8 large whole eggs, heavy cream, garlic, ground mustard, paprika, nutmeg, salt, and black pepper together in a large bowl. Set aside.
Line the bottom of your prepared baking dish with slices of bread. Top with 3/4 of the cheddar cheese, half of the Boursin cheese, and half of the ham. Top with another layer of bread slices. Pour the egg mixture on top, making sure to coat the bread evenly. Top with the remaining cheese, ham, and Boursin.
Cover the dish and refrigerate for at least 1 hour or up to 1 day.
When you are ready to bake, preheat the oven to 350 degrees F. Cover the dish with foil (if not already covered with foil) and bake for 45 minutes. After 45 minutes, remove the foil and bake uncovered for 20-25 minutes more. Remove from oven and test the center with a butter knife. If the butter knife is covered in liquid, the casserole needs more baking time. A little wetness with crumbs is OK. Let it rest for 15 minutes before serving.
While the casserole is baking, prepare the herb hollandaise. To a stainless steel bowl set over a simmering pot of water (making sure the bottom of the bottom of the bowl does not touch the water), combine the eggs, lemon juice, cayenne, and white pepper. Whisk constantly for 3-4 minutes or until the eggs double in volume and are nice and foamy. Very slowly, whisk in the butter. Continue whisking for another 4-5 minutes or until the sauce thickens to a point where you can coat the back of a spoon. Remove from heat immediately and stir in the herbs.
Serve the casserole topped with hollandaise and fresh parsley.
This casserole keeps very well in the fridge for 2-3 days after bake. Enjoy!