MAMA TERESA'S VEGETABLE SOUP

Steps:

• Cut a shallow X in bottom of each tomato with a sharp paring knife and blanch tomatoes in a 2- to 3-quart saucepan of boiling water 20 seconds. Transfer tomatoes with a slotted spoon to a cutting board and, when cool enough to handle, peel, beginning from scored end, with knife, then seed and chop.
• Heat oil in a 4-quart heavy pot over moderately high heat until hot but not smoking. Add onion and tomatoes, then reduce heat to moderate and cook, stirring occasionally, until onion is softened, 4 to 5 minutes.
• Meanwhile, peel potato and cut into 1/4-inch dice.
• Add potato to onion mixture along with zucchini and celery and cook, stirring occasionally, 5 minutes. Add water, herbs, sea salt, and pepper and bring to a boil, uncovered. Reduce heat and simmer, uncovered, stirring occasionally, until vegetables are very tender, 30 to 35 minutes. Remove from heat.
• Whisk together egg and cheese in a bowl until combined well, then add to soup in a stream, stirring. Divide among 4 bowls and serve with olive oil and cheese.