Pour the tomatoes into a food processor and pulse until chopped.
Heat oil and the red pepper flakes in a sauce pan over medium heat
Add the garlic and onions. Stir and cook the onions until tender (about 5 minutes)
Add the tomatoes and chicken stock. Bring to a boil and then reduce to a simmer (until slightly reduced) for about 30 minutes
Ladle the soup into a blender (or use an immersion blender) to puree until smooth. You could also strain the soup after the puree
Croutons
Drizzle bread with oil, toast in 375 degree oven until golden (about 5 minutes). Top with mozzarella and return to oven to melt the cheese (1-2 minutes)