Sauté Spinach: Heat olive oil in a pan. Add spinach and cook until wilted. Season with salt and pepper. Set aside.
Make Hollandaise: Whisk egg yolks and lemon juice in a bowl over simmering water. Slowly add melted butter while whisking until thick. Add a bit of warm water if too thick. Season with salt. Keep warm.
Poach Eggs: Bring water with a splash of vinegar to a light simmer. Crack eggs into small bowls, then slide into water. Poach for 3–4 minutes. Remove with a slotted spoon.
Toast Muffins: Toast the English muffins until golden and crisp.
Assemble: On each muffin half, layer sautéed spinach, smoked salmon, and a poached egg. Spoon over warm hollandaise sauce. Garnish with chives.