Preheat the oven to 375°F In a bowl, mix together half the balsamic vinegar, half the garlic flavoured oil, sugar, oregano and seasoning.
Place the onion wedges and garlic in a roasting tin, and spoon the balsamic mixture over. Turn the onions in the vinegar mixture until they are thoroughly coated. Bake in the oven for 15 minutes.
Turn the onions over, sprinkle with the sesame seeds and cook for a further 15 minutes or until the onions are soft and sticky and the seeds are golden. If the balsamic marinade begins to burn while the onions are roasting, add 1-2 tbsp hot water to the tin.
Whisk the remaining balsamic vinegar and oil to make a dressing. Tip the arugula into a large serving dish and drizzle with the dressing. Add the onions and garlic, and serve with warmed crusty bread.