In a large bowl, combine the ground beef (or lamb), breadcrumbs, Parmesan cheese, chopped parsley, onion, garlic, oregano, salt, pepper, and beaten egg. Mix until well combined.
Preheat your oven to 400°F (200°C). Form the mixture into small meatballs, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for about 20 minutes, or until they are browned and cooked through.
While the meatballs are baking, heat olive oil in a large skillet over medium heat. Add the chicken broth, lemon juice, and lemon zest. Bring to a gentle simmer.
If you prefer a thicker sauce, mix the cornstarch with a little cold water to create a slurry, and gradually whisk it into the simmering sauce until thickened.
Once the meatballs are done baking, carefully add them to the skillet with the lemon sauce. Cook for an additional 5 minutes, gently spooning the sauce over the meatballs to coat them well.
Remove from heat and serve the meatballs warm, garnished with lemon slices and extra parsley.
Notes
Optional cornstarch can be used to thicken the sauce.