[France] How to cook Beetroot and walnut salad (salade de betteraves aux noix)

Ingredients
- 3 baby red beetroots, trimmed and scrubbed
- 3 baby yellow beetroots, trimmed and scrubbed
- 2 tsp raspberry vinegar
- ½ tsp Dijon mustard
- 1½ tbsp walnut oil (see Note)
- salt and freshly ground black pepper
- 2 tbsp fresh goat's cheese
- 2 tbsp walnut halves, lightly toasted
- 2 tbsp chopped chives