Vegetable Biryani

2 tsp olive oil, plus a little extra for brushing 2 large onions, peeled and thinly sliced lengthways 2 garlic cloves, peeled and finely chopped 2.5 cm/1 inch piece fresh root ginger,peeled and finely Grated 1 small carrot, peeled and cut into sticks 1 small parsnips, peeled and diced 1 small sweet potato, peeled and