While the casserole is baking you can make the hollandaise sauce. Make sure the clarified butter is still liquid, and warm but not too hot. This is especially important if you clarified butter in advance.
Heat up a separate skillet. Add the vinegar and crushed peppercorns. Cook down until the vinegar is almost completely evaporated. Add the water.
Transfer everything to a metal mixing bowl. You need a bowl that will fit over one of your sauce pans and act as a double boiler.
Add the 6 egg yolks to the metal mixing bowl and whisk together.
Place the bowl over a small saucepan of boiling water. Turn the heat down and whisk until the eggs thicken up.
Take the bowl off of the heat and slowly drizzle in the butter while whisking. You almost want to add it a drop at a time at first. Too quickly and the emulsion will break. Add lemon and salt to taste. Add 2-3 tablespoons of water to thin the consistency of the sauce. Be careful and only add it a tablespoon at a time.
Pour the sauce over the hot casserole. You will want to serve this hot and eat right away. If you want to save the casserole for additional meals then you will want to leave the sauce off of the top - it does not keep.
Garnish with fresh chives or green onion.