Preheat the oven to 200°C. Lightly grease a large baking sheet with olive oil.
1. Vinaigrette Mix all the ingredients until well combined. Transfer to a jar or bottle and set aside.
2. Veggies Put the leeks on the prepared baking sheet, drizzle a little oil over and roast for 15 minutes. Remove from the oven and add the garlic and mushrooms, drizzle a little more oil over and roast for another 10 minutes.