Preheat the oven to 170°C/325?F/gas mark 3.
Peel and cut the onions in half lengthways and then into quarters and place into an ovenproof dish. Chop 4 of the garlic cloves finely and sprinkle on top of the onions together with the fresh thyme and sugar. Pour over the vinegar and half the olive oil and season well with salt and pepper. Place in the oven and cook for about one hour mixing every 15 minutes or so until golden brown and soft.
While cooking make the pesto. Place the parsley, not stalks, into a blender with 4/5 of the oil. Add the other garlic clove, anchovy and cheese. Season well and blend until a smooth paste is achieved, this should only take a minute.
Heat a non stick pan on the stove with the remaining oil and season the fillets of snapper well. Place into the pan, skin side down and hold each piece down well to prevent the skin and fillet curling when cooking. Once the skin is nice and golden brown, turn over and place in the oven to finish cooking for about 3 to 4 minutes.
Remove the onions and place onto the middle of each plate. Place the snapper fillets on the top and pour over the remaining juices from the red onion pan. Drizzle with the parsley pesto and serve.
Per serving: 295 Calories (kcal); 23g Total Fat; (67% calories from fat); 3g Protein; 22g Carbohydrate; 0mg Cholesterol; 8mg Sodium Food Exchanges: 0 Grain(Starch); 0 Lean Meat; 3 1/2 Vegetable; 0 Fruit; 4 1/2 Fat; 0 Other Carbohydrates