Peel and chop the celeriac into small chunks. Tip into a pan. Crush the cardamom pods in a pestle and mortar. Discard the husks. Tip the black seeds in with the celeriac. Cover the celeriac with boiling water. Add a pinch of salt. Cover and simmer for 20 mins.
Meanwhile, peel and finely slice the red onion. Zest and juice the orange. Finely chop the tomatoes.
Put half of the sliced onion into a bowl. Pour over half the orange juice. Add a pinch of salt and leave to marinate.
Set a frying pan over a high heat. Pour in 1 tsp oil. Add the beef strips. Cook and stir for 2-3 mins till browned. Using a slotted spoon, transfer to a bowl.
Keep the pan on the heat and add 1 tsp oil. Fry the remaining sliced onion for 3 mins till just softened. Add the tomatoes and cook for 3 mins till softened. Add most of the thyme leaves, the orange zest and remaining juice. Cook for 2 mins till bubbling.
Add the beef to the pan, along with any resting juices from the plate. Simmer for 4 mins.
Drain the celeriac, then tip back into the pan. Put the pan over a low-medium heat and mash the celeriac. By mashing on the heat, any excess water will evaporate. Season the mash well.
Toss the orangey onions through the rocket. Serve with the beef on top of the cardamom celeriac mash. Scatter over a few thyme leaves to garnish.