2leekswhite and light green parts, halved and thinly sliced into half circles
1poundbroccoli rabe, curly endive or escarolecleaned and coarsely chopped
Salt and freshly ground black pepper
freshly grated Pecorino Romano cheesefor topping soup
To make the meatballs:
In a large bowl stir together the first 7 ingredients (onion through Pecorino). Add beef, pork and several grinds of black pepper; stir until just combined or combine with your hands. (Don't overwork this mixture or your meatballs will be tough.)
With a small scoop or teaspoon, scoop out about 1 ½ teaspoons of meat mixture and place on a baking sheet; repeat with remaining mixture. Quickly roll each portion between your palms to form a meatball.
To make the soup:
Heat a large soup pot over medium heat. Add bacon; cook and stir until fat has rendered and bacon is nearly crisp. Add leeks; cook and stir until tender, about 2 minutes. Add stock and bring to boil.
Add meatballs and broccoli rabe or other green to boiling broth; reduce heat and simmer until meatballs are cooked through and greens are tender, 8 to 10 minutes. Season to taste with salt and pepper.
Ladle soup into bowls. Top each with a generous pinch of Pecorino Romano cheese and serve.