1. Preheat your oven to 190°C/375°F.

2. Combine the ground beef, salt, and black pepper in a large mixing bowl, mixing well with your hands or a wooden spoon until just combined. Be careful not to overmix.

3. Form the beef mixture into small balls, about 3.8 cm/1 1/2 inches in diameter, and place them on a baking sheet lined with parchment paper, leaving about 2.5 cm/1 inch of space between each meatball.

4. Heat the vegetable oil in a large pot over medium heat, add the diced onion, and cook until softened, about 5 minutes. Then add the diced leek and cook for an additional 2 minutes.

5. Add the diced tomatoes, tomato paste, flour, and vegetable broth to the pot, stirring well to combine. Bring the mixture to a boil, then reduce the heat to low and simmer for 10 minutes.

6. While the soup is simmering, bake the meatballs in the preheated oven for 12-15 minutes, or until cooked through.

7. Once the meatballs are done, add them to the pot with the soup and simmer for an additional 5 minutes to allow the flavors to meld.

8. Whisk together the heavy cream and shredded cheese in a small bowl until smooth, then stir the cheese mixture into the soup and simmer for an additional 2-3 minutes, or until the cheese is melted and the soup is heated through.

9. Serve the soup hot, garnished with fresh basil leaves and accompanied by crusty bread.