In a large frying pan heat the butter and oil. Add the frozen Birds Eye Onions and cook on high for 5 minutes, stirring constantly. Add the mushrooms and garlic and cook on a medium heat for 5 minutes.
Stir in the brown sugar and reduce heat; allow to caramelise for 15 minutes, stirring occasionally (liquid should be reduced and the onions turned a caramel colour). Remove from heat and set aside to cool.
Place the onion mixture in a food processor along with the balsamic vinegar, salt and pepper and process until chopped and still chunky.
To Serve: Spoon into serving dish and drizzle with olive oil. Serve with fresh crusty bread.