To make the mayonnaise.
Bring a small saucepan of water to the boil, add the egg and cook for 5 minutes. Cool the egg in a bowl of iced water.
Peel the egg and put in a medium bowl then break it up into small pieces with a fork. Whisk in the mustard then whisk in the oil, little by little, until it is emulsified. Stir in the lemon juice and pickled mustard stem and season with salt and white pepper.
Gently warm 1 teaspoon of the grapeseed oil in a saucepan over medium heat. Add the sausage and cook for 5 minutes, stirring frequently. Set aside to cool. Once cool, use a mortar and pestle to pound the sausage until it resembles rough crumbs.
Trim the tough ends off the asparagus. Heat a chargrilled pan or barbeque until very hot. Toss the asparagus in the remaining grapeseed oil and grill for 2 minutes on each side or until nicely charred.
Remove the asparagus from the grill, lightly season with sea salt and slice the spears into 3 on a sharp angle.
To serve, place a spoonful of mayonnaise on a serving plate and arrange the asparagus on top. Spoon more mayonnaise in and around the asparagus. Finish by sprinkling over the sausage crumb and lemon juice.