Preheat oven to 425°. Toss the vegetables with olive oil and season with 2 teaspoons salt and a pinch of white pepper. Place them on a large baking sheet in the oven. When vegetables are roasted but not too browned (about 15 minutes), remove from oven.
In a medium saucepot melt butter and add roasted vegetables along with 2 bay leaves. Add stock to cover the vegetables and add 1 tablespoon salt.
Bring to a boil then quickly reduce to a simmer. Simmer for 10 minutes.
Remove the bay leaves.
Puree soup in a blender, then add yogurt and cream. Quickly pulse soup to distribute the cream and yogurt. Optionally, strain the soup through a fine-mesh sieve for a more refined velvety-textured soup. Return to pot and season with salt to taste.
Serve in warm bowls topped with minced chives, a sprinkle of the hazelnuts and a few drops of hazelnut oil.