Heat the oven 180°C/350°F/gas mark 6.
First, make the dressing. Put all the ingredients in a small bowl or jug, season with salt and pepper and whisk vigorously to combine. Set aside while you make the salad.
Scatter the celeriac over a large baking tray, then the onions, then the garlic, rosemary, thyme and bay. Trickle one tablespoon of the olive oil over and season well with salt and pepper.
Place the baking tray in the hot oven and roast, turning the celeriac occasionally, for 35 minutes. Remove the tray from the oven and scatter over the seeds, then return to the oven for a further eight to 10 minutes, or until the celeriac is a little caramelized on the outside and tender in the middle. Remove from the oven and keep warm.
Season the steak all over with plenty of salt and pepper. Heat a medium–large frying pan over a high heat. Add the remaining olive oil and, when hot, add the steak. Cook for about one to two minutes on each side for rare; or two to three minutes on each side for medium. Lift the steak from the pan and keep warm.
Take a large, warmed serving platter and scatter over the roasted vegetables and seeds. Slice the skirt steak thickly across its grain and place this on and around the celeriac. Top with the salad leaves, then trickle over the dressing and bring to the table.