Bring a pan of water to the boil over a medium heat, add the egg, return to the boil, then turn the heat down to low and simmer for 10 minutes. Drain and refresh under cold water, then crack and peel the egg. Finely chop the cooked egg and leave to one side.
Divide the asparagus into four equal-sized bunches and tie each bunch with kitchen string to secure. Bring a pan of salted water to the boil over a medium heat, add the bunched asparagus and simmer for 5-8 minutes (depending on the size of your asparagus) until tender, but with just a little resistance when pierced with the top of a knife.
Grate the zest from the grapefruit, reserve a pinch and put the rest in a small bowl. Peel the grapefruit, then take out four segments, cut each one in half and leave to one side, juice the rest of the grapefruit, pressing it through a fine sieve into the bowl with the zest. Add the mustard and mayonnaise and whisk together until well blended, then whisk in the sunflower oil in a steady stream. Stir in the chopped egg and chervil and season with salt and pepper to taste.
Remove the asparagus from the pan and drain briefly on a plate lined with kitchen paper or a clean tea towel. Remove the string and put the asparagus onto four serving plates. Spoon the dressing over the top and finish with the reserved grapefruit segments and a sprinkling of the reserved zest.
Serve with griddled green asparagus and chargrilled bread.