Heat the milk over moderate heat in a medium pot. Put tomatoes, tomato paste, onions, salt and pepper, sugar, flour, butter, celery into a food processor and grind until smooth. Pour mixture into milk and raise heat to bring soup to a boil. Reduce heat and simmer 15 minutes. While soup cooks, make pesto and sammies.
Fill the food processor with watercress, loosely packed. Add hazelnuts, 1/2 of the oil and garlic cloves to the watercress in the processor. Add nutmeg, salt and pepper to the processor bowl, set lid in place and pulse grind the mixture into a thick paste. Add any remaining watercress to the processor and grind into the paste by pulsing the processor again. Transfer watercress paste to a bowl. Stir in the remaining oil and the cheese. Taste pesto sauce and adjust seasonings.
Heat a griddle over medium high heat. Drizzle the pan with extra-virgin olive oil and add the patties and cook 5 minutes on each side. Remove from the griddle and add another tablespoon extra-virgin olive oil and the peppers and onions. Season the peppers and onions and cook until just tender, 5 minutes. Remove them from the griddle. Wipe the griddle clean and reduce heat to medium low. Assemble sandwiches: place 4 slices of bread down, top with 1/4 each of the peppers onions, then a slice of cheese, a cooked sausage patty, another slice of cheese and then place bread on top. Add butter to griddle and cook sammy melts 2 or 3 minutes on each side until golden and cheese is melted. Press with spatula as sammies cook or set another heavy skillet on sammies to weigh them down.
Pour soup into shallow bowls and add a couple of rounded spoonfuls of pesto to each dish and stir it into the soup. Cut the patty melts corner to corner and dip in the soup as you eat.