Preheat the oven to 180ºC/gas 4. Beat the eggs with a little sea salt and black pepper. Heat the oil and butter in a 19cm ovenproof frying pan or cast-iron dish over a medium heat. Trim and add the asparagus for 5 minutes. Add the eggs, cook for 3 minutes then bake for 15 to 20 minutes, or until ... Line up the asparagus, trim off the woody ends, and place in a colander above the eggs, covered, to steam while the eggs cook. Meanwhile, pick the tarragon leaves. In a bowl, whisk the mustard with 6 tablespoons of extra virgin olive oil, adding it gradually, then pour in the soaking vinegar from the onion through a sieve, whisking constantly.