Purple Potato Vichyssoise with Roasted Garlic

Name: Purple Potato Vichyssoise with Roasted Garlic
Ingredients: 1 whole bulb garlic; 1 teaspoon olive oil; 1 tablespoon butter; 2 large leeks, white and light green parts only, coarsely chopped and thoroughly washed; 1 cooking onion, chopped; 3/4 cup dry white wine; 8 purple potatoes, or 4 medium Yukon gold or russet potatoes, peeled and sliced or diced; 6 cups vegetable broth or chicken stock; 1/2 teaspoon salt; 1/2 teaspoon white pepper; 4 sprigs fresh sage, or 1 tablespoon dried sage; 3 cups whole milk