Make the vinaigrette in a small bowl, combine the lemon juice, garlic, and fish sauce. Let it sit for a few minutes so that the garlic has time to give up some of its strength. Whisk in the oil, then add the coriander seeds.
Melt the butter in a 5-quart/4.7 liter pot over medium heat and sweat the leeks, adding a four finger pinch of salt. Add the stock and bring to a simmer, then reduce the heat to low. Add the carrots and potato and simmer gently for 5 minutes or so. Add the celery root and continue to cook until the vegetables are tender. Taste and adjust the seasoning with salt and pepper.
Add the beef, pressing it down into the stock so that it’s submerged. Raise the heat to medium-high and cook the beef just until rare, 1 to 2 minutes. Divide the leeks among 4 warm bowls. Distribute the root vegetables evenly, and top each serving with 3 pieces of beef. Stir the vinaigrette and spoon over the beef and vegetables. Garnish with the cilantro and serve.