Make the hollandaise sauce with an upright blender or immersion blender: Blend the yolks at high speed until combined. Drizzle in the melted butter until all is added and thickened.
Make the hollandaise sauce by hand: Whisk the egg yolks in a bowl, then drizzle in the melted butter while whisking until thickened.
Season the hollandaise with kosher salt and lemon juice, then set aside.
Prepare the spinach: Heat a skillet, add a small puddle of water, and cook the spinach until wilted. Press out excess water and set aside.
In the same skillet, heat butter, add shallots, and cook until golden. Return spinach to the skillet, season, and warm through.
Poach the eggs: Heat a saucepan with water and a splash of vinegar. Create a whirlpool and gently add the egg. Cook for 3 to 5 minutes until poached.
Toast the English muffins in the skillet with butter until golden brown.
Assemble: Place the bottom half of toasted muffins on a platter, add spinach, a poached egg, and hollandaise. Top with the muffin half and serve.
