Mix ground beef, breadcrumbs, Parmesan, egg, garlic, Italian seasoning, salt, and pepper. Form into 1-inch meatballs (about 16).
Heat olive oil in a large skillet over medium. Cook meatballs for 6-8 minutes, turning, until browned. Remove and set aside.
In the same skillet, heat olive oil; cook onion and garlic for 2-3 minutes. Add broth, lemon juice, lemon zest, salt, pepper, and chili flakes. Stir in cornstarch slurry; simmer for 2-3 minutes until thickened.
Return meatballs to skillet; simmer for 5-7 minutes until cooked (160°F).
Garnish with parsley, dill, and lemon slices. Serve hot.