Heat oven to low, about 170 deg F or 70 deg Celsius Heat a fairly deep oven dish.
Fry the onion in the oil, stirring, until translucent. Keep heat fairly high and stir often.
Just as onion bits are translucent and seem to start burning, turn heat down somewhat and add the balsamic vinegar. Stir well -- heat must be fairly high so that vinegar sort of evaporates.
Add the tomatoes (the actual weight of the tinned tomatoes is not all that important), the sugar, fish sauce, garlic, salt, Tabasco sauce (to taste!), wine and parsley.
Stir well and simmer through, and taste at this point: more sugar? More salt? A little more wine? It's up to you.
Add the mussels. Always make sure that frozen mussels have defrosted, been rinsed well, and then still check them for bits of broken shell and bits of beard, before adding to the sauce.
Stir through the sauce gently with a long-handled spoon, and let simmer gently for 4 minutes or so.
Pull off the heat, and ladle the mussels into the warmed oven dish. Pour over the sauce, which will be quite thin. Add the chopped spring onions (a small bunch, as they are usually sold, should give you 1 cup chopped).
With a long-handled spoon, lift and move the mussels and let some of the green onions slide into the liquid.
Cover with foil, keep warm until serving, and also warm soup plates. Then serve with lots of fresh crusty bread or baguette, and a large bowl of torn mixed green salad leaves, with vinaigrette dressing on the side. Won't serve more than 2 or 3!