In a large bowl, combine chicken, spinach, feta, grated onion, garlic, mustard, red pepper flakes, lemon zest, fresh dill, dried oregano, egg, breadcrumbs, 1 teaspoon salt, and black pepper. Roll the mixture into 1½ -inch meatballs. Refrigerate for 15 to 30 minutes.
Heat 2 tablespoons oil in a large Dutch oven or heavy-bottomed pan over medium-high heat. Add the meatballs in batches, without over-crowding the pan and sear for 3 minutes per side. Transfer the meatballs to a plate.
Heat the remaining 2 tablespoons of oil in the same pan used for the meatballs, over medium-high heat. Add the onion and sauté until translucent, about 3-4 minutes, stirring occasionally. Add the garlic and sauté for 30 seconds. Add the rice, chicken stock, lemon zest, dried oregano and remaining 1 teaspoon salt, stir well and bring to a light boil. Cover the pan with lid and simmer rice for 10 minutes. Fluff the rice with a fork, quickly add the meatballs on top of the rice, cover immediately and continue to cook for an additional 10 minutes, or until rice is tender.
Finish with fresh lemon juice, herbs, and cheese. Enjoy!
