Cut the monkfish into chunks and season lightly. Melt half the butter in a large saucepan and gently fry the onion until it is softened but not browned. Add the fish and cook, stirring, for 2 minutes. Remove the fish with a slotted spoon and add the garlic to the pan. Cook for 1 minute.
Sprinkle in the rice and fry gently for 1 minute. Add the wine and let it bubble until almost evaporated, then add the saffron and lemon thyme.
Add the stock over a medium heat, a ladleful at a time, stirring continuously. Allow each ladleful of stock to be absorbed before adding the next. Keep adding stock until the rice is cooked but still has a slight bite to it. This should take about 15–20 minutes.
Check the seasoning and stir in the fish. Heat through and serve immediately, scattered with grated Parmesan and chopped lemon thyme.