Preheat the oven to gas 3, 170°C, fan 150°C.
Cut the cherry tomatoes into 6 bunches and arrange on a baking tray. Drizzle with the olive oil and season. Roast for 25 minutes, then remove from the oven and drizzle over the balsamic vinegar. Set aside until needed.
For the rosti, put the grated potatoes in a sieve and rinse with cold water to get rid of the starch. Tip the mixture onto a clean tea towel, then roll it up tightly and squeeze to remove as much water as possible. The drier the potato, the crispier your rosti will be.
In a bowl, mix the potatoes with the onions, flour and egg, and some seasoning. Shape the potato mixture into 12 flat cakes. Heat two large frying pans and add 1⁄2 tbsp of butter and olive oil to each. Once the butter has melted, cook the potato cakes (in batches if needed) over a low heat, for 10 minutes each side, until golden. Reheat the rostis and tomatoes in the oven for 10 minutes before serving.
To serve, arrange two rosti on each plate and top with the chicory, Gorgonzola, roasted tomatoes and their juices. Garnish with the watercress and a scattering of toasted pinenuts.
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