Combine hollandaise with 2 teaspoons dill. Toast the English muffins and divide between 4 plates, top each English muffin with 2 slices smoked salmon, then a poached egg. Spoon dill hollandaise over each egg, garnish with extra dill and sliced scallions if desired and serve immediately.
Combine egg yolk, water, lemon juice, and a pinch of salt in the bottom of a cup that barely fits the head of an immersion blender. Melt butter in a small sauce pan over high heat, swirling constantly, until foaming subsides. Transfer butter to a 1 cup liquid measuring cup.
Place head of immersion blender into the bottom of the cup and turn it on. With the blender constantly running, slowly pour hot butter into cup. It should emulsify with the egg yolk and lemon juice. Continue pouring until all butter is added. Sauce should be thick and creamy. Season to taste with salt and a pinch of cayenne pepper or hot sauce (if desired). Serve immediately, or transfer to a small lidded pot and keep in a warm place for up to 1 hour before serving. Hollandaise cannot be cooled and reheated.
Brunch time! Switch up the classic eggs benedict by topping english muffin with smoked salmon and dill.