Grate the onion into a bowl. Add red wine vinegar, honey, 1/2 teaspoon mustard powder, 1/4 teaspoon ground ginger. Whisk together. Gradually whisk in the oil so that the dressing emulsifies and thickens. Add the poppy seeds and whisk again.
Peel the grapefruit and separate into segments. Rinse the rocket leaves and dry on paper towels. Slice the avocado into thin slices.
Toss the rocket with 1/3 of the dressing and scatter on a plate. Arrange the grapefruit and avocado slices prettily on top. Curl the prosciutto slices into cones and arrange in the centre. Spoon over more dressing as desired - you may not need it all.