Preheat the oven to 375 degrees F. Line a large baking sheet with parchment paper. Mix the onion, garlic and olive oil in a bowl. Spread onion mixture in a thin layer on the baking sheet. Roast 15 minutes, and then stir the onion mixture, ensuring it ends up back in a thin layer. Roast the onions 15 minutes more, or until lightly golden.
Cool onions to room temperature, and then transfer to a medium bowl (make sure you get all of the onions). Mix in the remaining ingredients, except garnish. Cover and refrigerate dip until ready to serve. Can be made up to a day in advance of serving. If desired, just before serving, garnish top of the dip with a little thinly sliced green onion.