In a spacious mixing bowl, combine the ground beef (or lamb), breadcrumbs, grated Parmesan, chopped parsley, diced onion, minced garlic, dried oregano, sea salt, black pepper, and the lightly beaten egg. Use your hands or a fork to mix until all ingredients are thoroughly combined and sticky.
Preheat your oven to 400°F (200°C). Shape the mixture into small meatballs, about 1 inch in diameter, ensuring they are uniform in size for even cooking. Place them on a baking sheet lined with parchment paper, spaced apart to allow for even browning.
Bake the meatballs in the preheated oven for approximately 20 minutes or until they are golden brown and cooked through, checking that the internal temperature reaches 160°F (70°C).
While the meatballs bake, heat the extra virgin olive oil in a large skillet over medium heat. Pour in the chicken broth, followed by the freshly squeezed lemon juice and lemon zest. Bring the mixture to a gentle simmer, allowing the flavors to meld.
If you desire a luscious, thickened sauce, dissolve the cornstarch in a small bowl with a tablespoon of cold water to create a slurry. Gradually whisk this into your simmering sauce, stirring continuously until it thickens to your preferred consistency.
Once the meatballs are baked, gently transfer them into the skillet with the lemon sauce. Cook for an additional 5 minutes on medium-low heat, carefully spooning the sauce over the meatballs to ensure they are well-coated and absorb the bright lemon flavor.
Remove the skillet from the heat and serve the meatballs immediately, garnished with bright lemon slices and a sprinkle of fresh parsley for a pop of color.