Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the beetroot and slice into wedges. Pop them in a bowl. Add 1 tbsp olive oil with a pinch of salt and pepper. Turn the beetroot a few times to coat them.
Arrange the beetroot on a baking tray. Slide into the oven and roast for 20-25 mins till tender.
While the beetroot roast, put a dry frying pan on a medium heat. Add the walnuts. Toast for 3-4 mins till browned and nutty smelling. Tip out onto a board and leave to cool for a few mins, then roughly chop them. Tip into a large bowl.
Add the caraway seeds to the pan. Toast for 1 min till they start to pop and smell aromatic. Tip into the bowl with the walnuts.
Pick any really thick stalks out of the watercress. Add the tender sprigs and leaves to the walnuts.
Slice the plums into wedges, discarding the stone, and add to the bowl.
Whisk the Dijon mustard with the red wine vinegar and 1 tbsp olive oil. Add a pinch of salt and pepper, taste and adjust the seasoning till you like it. Pour half the dressing into the bowl and toss to mix.
Arrange the dressed salad on two serving plates. Top the salad with the roast beetroot wedges. Drizzle over the remaining dressing and serve.