Kohlrabi and gorgonzola gratin

sunflower oil

medium onion, peeled and thinly sliced

vegetable stock

double cream

Dijon mustard

parmesan, grated

garlic , peeled and crushed

large sage leaves, finely chopped

Salt and freshly ground black pepper

medium-large kohlrabi, peeled and cut into 2-3mm-thick rounds

gorgonzola, broken into 1cm pieces

panko breadcrumbs