In a large saucepan, melt the butter over medium-high heat. Add the prosciutto, onions, celery, carrots, garlic, and bay leaf and cook, stirring, until the onions are soft and fragrant, about 3 minutes. Add the tomato paste and cook, stirring, for 2 minutes. Stir in the flour, and cook, stirring, for 1 minute. Add the tomatoes and stir well. Add the stock and bring to a boil. Reduce the heat and simmer, stirring occasionally, for 20 minutes. Add the cream and cook for 3 minutes.
Remove from the heat and discard the bay leaf.
If you like a smooth soup you may use a hand-held immersion blender, or in batches in a food processor, puree the soup. Season, to taste, with salt and pepper.
In a small saucepan, caramelize the sugar over low heat. Add the vinegar and stir to dissolve the sugar. Add to the soup a teaspoon at a time to balance the acidity.
To serve, ladle the soup into bowls and arrange the sliced sandwiches around the sides. Garnish with the herbs and chopped tortillas and serve.
Yield: 4 servings