Soup: Peel onions and cut in half lengthwise. Thinly slice crosswise. Trim leeks and cut in half lengthwise. Slice crosswise in 1/4-inch slices, using 3 inches of pale green. Place leeks in cold water and rinse to remove dirt. Drain. Reserve onions and leeks.

Dice pancetta into 1/4-inch pieces. Heat olive oil in large soup pot over medium-high heat. Add pancetta and cook until some of the fat has been rendered, about 5 minutes. Add onions and leeks. Sauté 20 minutes or until tender, stirring occasionally. Add stock and simmer 30 minutes.

Stir vinegar, wine, salt and pepper into soup. Cook until thoroughly heated.

Bread: Toast bread slices and rub with peeled garlic clove halves.

Final Preparation: Spoon soup into soup bowls. Float bread slices in soup. Pare 4 or 5 shavings of the Parmesan cheese on top of each serving. Garnish with chopped parsley and serve immediately.