Using a large pot or Dutch oven, sauté onions and garlic in oil until tender.
Add tomato paste and stir for another minute – this helps to cook the ‘raw’ flavor from the tomato paste.
Add the remaining ingredients and simmer for 20 minutes, season to taste with salt and pepper.
Remove the rosemary sprig and puree the soup using a food process, blender, or immersion blender, until smooth. Return to pot to keep warm.
For croutons, slice fresh focaccia, or chop into cubes and fry in a bit of oil until all sides are crunchy and golden.
To serve, pour soup into bowls, top with focaccia croutons and lightly drizzle with a bit of oil.