Set a non-reactive pot over medium heat and melt the butter. Then add the onions, garlic and oregano. Stir with a wooden spoon and sauté the aromatics for about 7 minutes until the onions turn translucent and slightly charred.
Add the tomato paste, celery seed and sauté for another 3 minutes.
Add the tomatoes and chicken stock and turn down the heat to low. Cover the pot with a lid and simmer for about 40 minutes.
When the soup is done cooking, transfer to a blender or use an immersion blender in the pot. Add the garlic confit and mascarpone and blend until smooth.
Garnish each bowl of soup with basil, mascarpone and a drizzle of the garlic confit oil. Serve immediately with your favorite grilled cheese. Enjoy!