1) Prepare potatoes according to package by cooking in a nonstick skillet over medium heat with a drizzle of oil. Brown the hash browns well on one side, then press into rounds and continue to cook until the rounds are crispy on both sides, about 6-7 minutes per side. Keep potato rounds warm until ready to serve.
2) For sauce, mix yolk, water, lemon, salt, and vinegar in a large bowl. Place bowl over simmering water bath. Be sure bowl doesn’t touch water.
3) Whisk until egg mixture is frothy and hot. It should at least double in volume and be slightly steaming.
4) Slowly whisk in melted butter until it forms a smooth sauce. If the sauce gets to thick, add a bit more water and whisk.
5) Poach eggs in lightly simmering water with added vinegar. Cook them for about 2 1/2 minutes. Drain eggs with a slotted spoon and pat dry on a paper towel.
6) Plate hash browns topped with seared Canadian bacon or ham while eggs cook. Then immediately top with eggs and sauce and serve right away garnished with scallions!