Poach the fish fillet in gently simmering salted water for 10-12 minutes, until it flakes easily. Let cool completely, then break into two-finger-wide pieces.
Cook the cauliflower and green beans separately in salted water until tender (cauliflower about 8 minutes, green beans about 10 minutes), cook the peas for 4-5 minutes, then drain and cool all of them.
For the poached eggs, bring water and vinegar to a boil in a pot, then crack the eggs one at a time into the simmering water and cook for about 3 minutes, until the white is set but the yolk stays soft. Lift out with a slotted spoon and transfer to cold water.
Arrange the fish pieces on a large platter, with the cauliflower, green beans, peas and tomato slices around them, and a poached egg on each plate.
Mix the oil with the pepper and the vinegar for the dressing, and drizzle over the whole platter.
Mix the grated onion, parsley and tarragon together, and scatter over the top.
Serve cold or lightly chilled, as a starter or light main course.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.