LEMON AND FENNEL RISOTTO

1 fennel bulb (about 330g) • 1.2 litres vegetable stock • 1 Sorrento lemon, zest and juice • 1 tbsp olive oil • 50g unsalted butter • 1 onion, finely chopped • 1 celery stick, trimmed and finely chopped • 350g arborio risotto rice • 150ml white wine • 60g pecorino, finely grated