Peel and cut potatoes into 1/2 inch cubes. Cut celery into 1/2 inch pieces. Core, blanch, peel and cut tomatoes into 1/2 inch chunks. Cut marrow into 1/4 inch slices. In a large saucepan, heat oil and brown garlic. Discard garlic. Add potatoes, stir to coat , and cook for about 5 minutes. Add onion, celery, parsley, and oregano, and cook until onions are softened. Add stock, tomatoes, marrow, and salt and bring to a boil. Reduce heat and simmer until zucchini is tender. Pour soup into individual bowls and sprinkle freshly ground pepper and 1 teaspoon Parmesan cheese over each serving. Serve hot.