Heat one tablespoon of olive oil in a non-stick frying pan and gently fry the onion until soft but not coloured.
Add the mustard and cream and simmer for 2-3 minutes. Season, to taste, with salt and freshly ground black pepper and whisk in the butter.
Stir in the sliced potatoes and chives.
Drizzle the asparagus spears with the remaining olive oil and heat a griddle pan. Griddle the asparagus for 3-4 minutes, or until just tender and charred.
Bring a saucepan of salted water to the boil and crack in the egg. Reduce to a simmer and poach the egg for four minutes. Remove from the pan with a slotted spoon and drain on kitchen paper.
To serve, place the potato salad onto a plate and top with the asparagus and poached egg.