Coarsely grate carrots, parsnip, and onions. Combine with cold water to cover in soup kettle and simmer 10 minutes. Add fish, cover, and simmer 20 minutes longer. Remove kettle from heat and cool. Remove fish, discard skin and bones, and cut meat into small pieces. Return fish to kettle. Add cucumbers, chives, parsley, dill, and lemon juice. Season with salt and pepper. Chill thoroughly. Garnish with additional fresh dill and a slice of lemon.