Toast the English muffins until golden brown. Spread with a little butter and keep warm.
Step 2
To make the Hollandaise-style sauce, whisk the egg yolk and lemon juice together in a heatproof bowl. Place the bowl over a pan of gently simmering water (do not let the bowl touch the water). Whisk constantly while slowly drizzling in the melted butter until the sauce thickens. Season with salt and pepper. Remove from heat and keep warm.
Step 3
Bring a saucepan of water to a gentle simmer. Crack each egg into a small bowl, then gently slide into the water. Poach for 3–4 minutes, or until the whites are set and the yolks are still runny. Remove with a slotted spoon and drain on kitchen paper.
Step 4
In a dry frying pan over medium heat, warm the mortadella slices for about 1 minute on each side, until just heated through and slightly crisp at the edges.
Step 5
To assemble, place the toasted muffin halves on serving plates. Top each with a slice of mortadella, a poached egg, and a generous spoonful of Hollandaise-style sauce. Garnish with chopped chives, if using. Serve immediately.