How to Make It
Bring the beef stock to a boil in a large saucepan. Add the leeks, cover and simmer over low heat, turning a few times, until tender, about 5 minutes. Using a slotted spoon, transfer the leeks to a large glass baking dish and cover with foil. Repeat with the remaining vegetables, cooking the pearl onions and carrots for 12 minutes, then the broccoli for 4 minutes and the zucchini for 2 minutes. Discard the strings from the leeks and cut them crosswise into 1-inch lengths, keeping the pieces intact. Re-cover the baking dish with foil. Keep the stock covered.
In a small bowl, combine the parsley with the chives, vinegar, water and tarragon. Blend in the olive oil and season with salt and pepper.
Preheat the oven to 325°. Bring the stock back to a simmer. Add the steaks, cover and cook over low heat for 3 minutes. Turn the steaks and continue cooking until medium rare, about 3 minutes longer. Meanwhile, transfer the warm vegetables to the oven for about 3 minutes, or until warm.
Arrange the vegetables on the plates. Transfer the steaks to the plates and ladle a few tablespoons of the stock over them. Season generously with salt and pepper. Give the herb dressing a stir and spoon it over all. Serve at once.