In a heavy saucepan, heat the butter over medium-high heat. Add the onion and garlic and saute for 1 minute. When the butter begins to bubble, add the flour and cook, stirring, for 1 minute. Gradually whisk in the milk and while whisking constantly, bring to a boil, about 3 minutes. Add 1/4 teaspoon salt, 1/4 teaspoon white pepper and nutmeg. Once the sauce has thickened, remove it from the heat and continue to whisk for 1 minute longer. Add the spinach in batches until wilted and combined. Remove the pan from the heat and reserve.
Bring 3 cups of water to a boil with the salt and vinegar in a small saucepan over high heat.
Crack an egg into a cup and then slide the egg gently into the water. Repeat with the remaining eggs. When the water returns to a boil, reduce the heat to low and simmer until the eggs are set. Watch carefully and remove the eggs when the yolks are still soft, about 2 to 2 1/2 minutes. Drain on paper towels.
To serve, top the toasted muffin half with the creamed spinach, then poached the eggs and the choron sauce (recipe below).
For Choron Sauce:
In a saucepan, combine the wine, shallots, tarragon, and tomato paste. Bring the liquid to a boil and reduce to 1 tablespoon. Cool.
In a heat-safe bowl set over a pot of simmering water. Cook's Note: Be careful not to let the bottom of the bowl touch the water. Whisk the egg yolks with the lemon juice, and water until pale yellow in color. Season the egg yolks with salt and freshly ground black pepper. Remove the bowl from the pot and, whisking vigorously, add the melted butter 1 tablespoon at a time, whisking until all the butter is incorporated. Whisk in the cooled red wine-tomato paste mixture. Keep the sauce warm until needed.