Make the hollandaise
To make the hollandaise, first make the shallot reduction. Put the shallot in a very small pan or skillet and cover with the vinegar. Cook over medium-high heat, adjusting the heat as needed, until all the liquid is cooked away, 5 to 6 minutes. Remove the pan from the heat and set the shallot reduction aside.
Set up a saucepan with 2 or 3 inches of water and bring it to just under a simmer over medium heat. Choose a saucepan that will be large and wide enough to hold a metal bowl and function as a “water bath.”
In the metal bowl that you have chosen for your water bath (but not over the heat), using a whisk, whisk the egg yolks lightly together. Lightly whisk in the hot water (you can measure it out from the water in the saucepan), then set the bowl over the saucepan of barely simmering water. (The hot water should not be touching the bottom of the bowl.) Use your whisk to whisk the yolks in a back-and-forth motion. Don’t whisk in any air and don’t get the eggs up the sides of the bowl, where they will scramble and cook. Be sure the water in the saucepan is hot but barely at a simmer. Turn the heat down or turn it off if you think the yolks are in danger of curdling. Keep steadily whisking the yolks back and forth until very thick (but not scrambled or curdled), 3 to 4 minutes.
Remove the bowl from the heat and whisk in the shallot reduction. Meanwhile, melt the butter in a small saucepan over medium heat. Transfer the butter to a Pyrex liquid measuring cup to make pouring easier. Add the hot butter to the eggs very gradually, whisking constantly (you don’t have to do the careful back-and-forth whisking here; just whisk vigorously and steadily) until all the butter is added. (Here are some tips for whisking in the butter: Make a cradle with a kitchen towel to steady your bowl as you add the melted butter. Even better, find someone to help you. One person can hold the bowl and the other can whisk, and you can trade off so your arms don’t get so tired!)
When you are finished adding the butter, if your hollandaise is thicker than you like it, whisk in a tablespoon or more of hot water. Add the dill and lemon juice and season to taste with salt and pepper. Keep the hollandaise warm while you finish the Eggs Benedict.
Make the eggs Benedict:
Put the 2 Tbs. butter in a large sauté pan over medium heat. Add the
spinach leaves and cook gently for a few minutes, just until they begin
to wilt. Push the spinach to one side of the pan and add the crab. Heat
the crab very gently for a moment, then remove the pan from the heat.
Put the split, toasted, and buttered English muffins on 4 warm plates.
Top each muffin half with some of the spinach, then with the warm
crabmeat. Then top each muffin half with a poached egg. Ladle
hollandaise over each egg, dividing it evenly, garnish with chopped
dill, and serve immediately.
For vegetarian eggs Benedict, omit the crab and add some sautéed wild
mushrooms to each muffin half, on top of the wilted spinach.