Heat your oven to 200°C/Fan 180°C/Gas 6. Slice the roots off the celeriac on the diagonal, so you leave as much flesh behind as possible. Peel off the skin and chop the celeriac into bite-size chunks. Rinse and scrub the carrot and Jerusalem artichokes, and chop into pieces around the same size as the celeriac chunks.
Tumble the chopped veg into a large roasting tin. Drizzle with 1 tbsp olive oil and sprinkle over a good pinch of salt and pepper. Toss till the veg are evenly coated, then slide into the oven and roast for 30 mins, turning halfway, till everything is golden and tender.
While the veg roast, peel the red onion and thinly slice it. Peel the garlic and crush or finely chop it. Pop both into a bowl. Pour over 2 tbsp red wine vinegar and sprinkle in a little salt and pepper. Scrunch together with your hands and set aside for 15–20 mins to lightly pickle.
Drain and rinse the chickpeas and lentils. Shake dry and tip into a large bowl. Pick the mint leaves off their sprigs and finely chop them.
Scoop the roast roots into the bowl with the chickpeas and lentils. Add the chopped mint and pickled onions along with their vinegary liquid. Add the rocket. Pour in 2 tbsp oil and toss to combine. Have a taste and add more salt or pepper if needed, then pile onto a couple of warm plates and serve.