Preheat oven to 425°F. Line a sheet pan with parchment and lightly brush with olive oil.
In a bowl, gently mix the ground chicken, egg, Parmesan, breadcrumbs, salt, oregano (or Italian seasoning), and pepper just until combined. Do not overmix.
Roll into small, 2-teaspoon-sized meatballs and place on the prepared sheet pan with space between them. Lightly brush or drizzle the tops with olive oil.
Roast 10–12 minutes, until cooked through and lightly golden. Broil 1–2 minutes at the end if you want extra color.
In a large pot over medium heat, add olive oil. Add the leeks with a generous pinch of salt and cook 8–10 minutes until soft and silky but not browned.
Add the grated garlic and cook 30 seconds, just until fragrant.
Deglaze the pot with the juice of 1 lemon, scraping up any bits from the bottom. Let it simmer for about 1 minute.
Pour in 12 cups chicken stock or broth and bring to a gentle simmer.
Add the pastina and cook 5–6 minutes, stirring occasionally so it doesn’t stick.
Turn off the heat and stir in the juice of 1 lemon. Taste and season with salt and pepper as needed. Add more lemon if you want it brighter.
Add the roasted meatballs to the pot or divide them into bowls and ladle the soup over top.
Finish with freshly grated Parmesan and extra lemon at the table.
Pro tip: this soup thickens as it sits because of the pastina. When reheating, loosen with a splash of broth or water and adjust lemon again before serving.