Start water boiling is a tall pan for the poached eggs. Add to this the vinegar and salt.
For sauce, whisk together yogurt, egg yolks, lemon juice and zest in a double boiler — heat and stir for 15 minutes until thickened. Pull off burner and whisk in mustard, salt, pepper and dill. Set aside and keep warm.
In a sauce pan, saute the spinach in the butter until wilted — about 1-2 minutes. Set aside.
Toast English muffins.
Poach eggs one at a time. For each egg, crack it in a small bowl (makes it easier to add to the water). Turn heat on the water so the water is just brely bubbling. Stir the boiling water in a clockwise manner, creating a little vortex in the center. GENTLY add egg to the vortex. DO NOT TOUCH IT. Let the egg set up in the swirl. Cook egg for 1 1/2 minutes. Remove with slotted spoon to a plate with a paper towel.
Assemble your dish. For each, lay 1/2 an English muffin on a plate face up. On top, add 1/4 of the spinach, 1 poached egg and a few spoons of the Hollandaise sauce. Top with a sprinkle of fresh dill.