Heat your oven to 180°C/Fan 160°C/Gas 4. Scrub the potatoes and chop them into 1 cm cubes. Place on a baking tray. Drizzle with 1 tbsp olive oil. Sprinkle with sea salt and pepper. Toss to mix. Roast for 15 mins.
After 15 mins, stir the tomatoes into the potatoes and scatter with about 10 of the sage leaves. Roast for another 15 mins.
While the veg roast, halve the plums. Scoop out the stones. Roughly chop them. Roughly chop the remaining sage leaves.
Place the plums and sage in a small pan with 1 tbsp sugar and 2 tbsp water. Cover. Heat gently for around 5 mins till softened. If it’s too runny, increase the heat to dry off extra liquid, or add an extra splash of water if the sauce needs it. Set aside, lid on, to keep warm once you have the consistency you want.
While the plums simmer, brush the pork steaks with ½ tbsp olive oil and season with salt and pepper.
Heat a frying pan for 2 mins till hot. Add the pork and cook for 10 mins, turning occasionally, till cooked through – to check, insert a skewer into a pork steak and any juices that run out should be clear. Set aside on a plate or board and loosely cover with foil. Let the steaks rest for a few mins.
Roughly chop the watercress. Take the veg out of the oven and stir in the watercress. Serve the steaks with the roast veg and the plum sauce.