Heat oven to 200C/180C fan/gas 6 and put the beetroot in a roasting tin with 1 tbsp oil, 1 tbsp vinegar and plenty of seasoning. Roast for 8-10 mins until sticky. Meanwhile, bring a pan of salted water to the boil, add the green beans, cook for 1 min, then drain. Toss in with the beetroot and roast for a further 5 mins.
Meanwhile, make the dressing by combining the remaining oil and vinegar in a small bowl, and season well. Put the mixed leaves and cucumber in a bowl and toss together with a little dressing.
Dip both cheese halves in the chopped walnuts so the tops are covered, then pop in the roasting tin with the beetroot and green beans for a few mins, to soften and toast the nuts.
Pile the salad onto plates and serve with the remaining dressing drizzled over the top.