Peel potatoes and cut into small cubes. Cut carrots and capsicum into small cubes.
Bring stock to a boil in a large saucepan. Add potatoes, carrots and beans. Cook covered over medium heat for 5 minutes. Add capsicum and peas. Cook, covered for another 5 minutes. Transfer vegetables to a sieve then leave to cool.
Mayonnaise Make sure all the ingredients and the bowl are at room temperature to prevent curdling. Whisk egg yolk, mustard powder and a generous pinch of salt until creamy. Add oil in a thin stream whisking until mixture thickens. Add 1 tablespoon lemon juice, salt and pepper to taste. Add 2 tablespoons caper brine. Taste, adding more brine if liked.
Place vegetables and capers in a serving bowl. Spoon on mayonnaise, stirring to coat. Place in refrigerator 1 hour.
Just before serving, add more salt, pepper and lemon juice to taste. Peel eggs; slice into wedges. Add to salad. Sprinkle with parsley.