For the soup, heat a soup pot over medium-low to medium heat with oil, 3 turns of the pan. Add onions, celery, garlic and chili flakes, season with a little salt and cook gently for 15 minutes to soften, stirring frequently. Add stock and raise heat to medium-high. Add tomatoes and break them up with wooden spoon; add passata and simmer 35-40 minutes to break down tomatoes. Purée soup with an immersion blender or in batches in a blender until smooth; adjust salt to taste.
For the sandwiches, stir together the butter, EVOO, garlic, parsley and grated cheese. Lightly butter 1 side of each slice of bread then build cheese sandwiches using both cheeses, with the buttered sides of the bread facing out. Grill sandwiches on a hot griddle over medium heat to deep golden.