Place the fish stock in a saucepan and bring to simmer while you get the risotto going. By the time you need the stock, it should be simmering.
Heat 2 tbsp of the butter and olive oil in a heavy based wide pan on medium heat.
Gently sauté the onions for about a minute or two until slightly soft, just golden.
Tip the rice in and coat thoroughly with all the lovely onion-butter mix.
Add the wine and stir well. As we only have a small amount of wine, it will be absorbed very quickly. As mentioned, skip the wine if you like and proceed to the next step.
Add 2 ladlefuls of the simmering stock and stir the contents of the pan. Keep stirring until the stock is almost all absorbed, then add another 2 ladlefuls. Keep repeating this, add, stir until almost absorbed until you are left with about 2 ladlefuls of stock. This will take about 15-20 minutes.
After you add the final 2 ladlefuls in step 5 (with some left in the pot), tip all the salmon pieces in.
Stir everything up thoroughly to cook the salmon as the stock is getting absorbed, about a minute.
When the stock is almost gone, add the spinach leaves, stir for about 30 seconds for them to wilt. Add a final ladle of stock and stir until absorbed. Check your rice, it should be cooked with just a slight bite to it. If it's not quite cooked, add another ladleful of stock.
Finally, when the rice is done, add the pesto in, stir for about 1 minute to heat right through and turn the heat off.
Stir the parsley in.
Add the parmesan in (if using), the half tbsp of butter, stir, cover and let rest for 2 minutes while you warm the plates up and get your cutlery.
Serve immediately with some lemon juice, more parmesan, parsley and freshly ground black pepper.