Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and slice into thin rounds. Place in a bowl. Add 1 tbsp olive oil and some salt and pepper. Turn to coat in the oil.
Line a baking tray with baking paper. Spread the potato slices out on the tray. Bake for 30 mins, turning the potatoes over after 15 mins.
In a large bowl, whisk together 2 tsp Dijon mustard and 1 tbsp balsamic vinegar. Add a pinch of salt. Peel the beetroot and coarsely grate it into the bowl. Peel and finely chop the onion and add that too. Mix together to coat in the dressing. Set aside for later.
Add 2 tsp of the coriander seeds to a dry frying pan. Toast over a medium heat for 2-3 mins till popping and fragrant smelling. Tip into a mortar and crush with a pestle till you have a coarse powder. No pestle and mortar? Tip into a bowl and use the bottom of a jam jar to crush them.
When the potatoes have cooked for 25 mins, rub each steak with 1 tsp oil. Season with salt and pepper and coat in the crushed coriander seeds.
Warm the frying pan you used to toast the coriander seeds over a high heat. When hot, carefully add the steaks and cook for 1 min on each side for rare, 2 mins on each side for medium and 3 mins on each side for well done. Remove from the pan and transfer to a plate. Cover loosely with foil to rest.
Remove the potatoes from the oven and divide between 2 plates. Place a coriander steak on each plate along with a large dollop of the beetroot relish. Serve with a handful of peppery salad leaves.