Blanch the onions in boiling water for 5 minutes. Drain and leave to cool for 5 minutes.
Meanwhile, heat 1 tablespoon olive oil in a large saucepan, cook the spinach until wilted and then roughly chop. Place in a bowl with the ricotta, egg yolk, thyme, Fontina and 20 g (3/4 oz) of the grated Parmesan.
Slice off the top of each onion and remove the middle sections with a fork.
Spoon the cheese mixture into the onions and place them in a roasting tin. Sprinkle over the remaining grated Parmesan, dot with butter and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes, until the cheese is bubbling and golden.
Serve on a bed of rocket leaves, drizzled with balsamic syrup.