Heat a large frying pan over a medium heat. Add 15 g unsalted butter. Once melted, add 280 g fresh spinach with a pinch of salt and some freshly ground black pepper. Cook, stirring occasionally, until the spinach has wilted, about 3-5 minutes. Transfer it to a colander, press out any excess water and set aside.
Meanwhile, make the cheese sauce (Mornay). Melt 15 g unsalted butter in a medium saucepan over a medium heat. Stir in 8 g plain flour and cook for 1 minute to make a roux. Gradually whisk in 240 ml whole milk until smooth. Bring to a gentle simmer and cook, stirring occasionally, until the sauce thickens, about 5-7 minutes. Remove from the heat and stir in 50 g grated Gruyère or Parmesan cheese, a pinch of ground nutmeg and season with salt and pepper. Keep warm.
For the hollandaise, set a heatproof bowl over a pan of gently simmering water to create a bain-marie. In the bowl, whisk 3 egg yolks with 15 ml fresh lemon juice and a pinch of salt until frothy. Gradually drizzle in 115 g melted unsalted butter in a thin, steady stream, whisking constantly until the sauce is thick and pourable, 5-10 minutes. Stir in a pinch of cayenne pepper and keep warm.
To poach the eggs, fill a large saucepan with about 8 cm of water and add 15 ml white wine or cider vinegar. Bring to a gentle simmer. Crack each of the 6 eggs into a small bowl and gently slide them into the water. Poach for 3-4 minutes, until the whites are set but the yolks remain runny. Use a slotted spoon to transfer the eggs to kitchen paper to drain.
Meanwhile, split and toast the 3 English muffins.
To assemble, place some wilted spinach on each toasted muffin half, top with a poached egg, spoon over the cheese sauce and finish with a drizzle of hollandaise. Garnish with chopped fresh chives or a sprinkling of paprika, if desired. Serve immediately.