Finely chop half the chives and cut the remaining half into 3cm (1in) lengths. Slice the celeriac into fine matchsticks using a mandoline, food processor with a julienne attachment or hand-held julienne tool.
Put the celeriac, crème fraîche, lemon juice, mustard, finely chopped chives, salt and pepper in a bowl and mix. Divide between 6 plates with the toast alongside. Drape the prosciutto over in swirls. Top with the remaining chives.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.