1. In a food processor, pulse onion, garlic, lemon zest and half of the rosemary until finely chopped.
2. Combine mixture with turkey, breadcrumbs, cheese, salt and pepper. Shape the mixture into 12 2-tablespoon meatball. Roll lightly in flour to coat and save remaining flour.
3. Heat oil in a large, nonstick skillet over medium heat. Add meatballs and cook until brown, 3 to 5 minutes. Transfer to a plate.
4. Add 1/4 cup of broth to the skillet, increase heat to medium-high and cook until almost evaporated, scraping in browned bits. Add remaining broth and bring to a boil. Reduce to a simmer and return the meatballs to the pan, along with the remaining rosemary. Cover and simmer until the meatballs are cooked through, 8 to 10 minutes. Remove the meatballs to a serving bowl.
5. Bring the sauce to a boil and cook until reduced to 1 cup. Whisk lemon juice and 1 tablespoon of the reserved flour in a small bowl. Whisking vigorously, add the flour mixture to the sauce. Simmer, whisking, until slightly thickened. Strain the sauce through a fine-mesh sieve and serve over the meatballs.