Using your hands or a large spoon, mix together 1 pound lean or extra lean ground beef, 1 pound ground turkey, 1 cup panko, 1 large egg, 1/2 cup crumbled feta cheese, 3 Tablespoons chopped parsley, 2 Tablespoons tomato paste, the minced garlic, 1/2 teaspoon kosher salt, and 1/4 teaspoon red pepper flakes. Mix only until the ingredients are well-distributed - don't overmix.
Scoop the meatball mixture into 2 Tablespoon to 1/4 cup portions (depending on how large you like your meatballs), and roll the mixture between your palms to form a ball. Repeat this process until all of the meat is used up.
Heat 1 Tablespoon olive oil in a large skillet over medium-high heat. Add the meatballs to the skillet and cook until browned on one side. (Depending on the size of your skillet and your meatballs, you may have to brown them in two batches). Turn all of the meatballs and cook until browned on that side. Repeat the turning process until the meatballs are browned on all sides. Reduce the heat to medium if at any time the meatballs are browning too quickly or burning.
Move the meatballs onto a plate and set aside for now. Leave any remaining oil in the skillet.
Tomato Sauce
Reduce the heat to medium and add the minced onions and 1 teaspoon kosher salt to the skillet. Cook the onions until soft, about 2-3 minutes. Stir in 2 Tablespoons tomato paste, the minced garlic, 1 teaspoon dried oregano, and 1/4 teaspoon red pepper flakes, and cook until fragrant, about 1 minute.
Slowly pour 2-4 Tablespoons water into the pan, scraping up any browned bits stuck to the pan while pouring. Once you have the pan deglazed, stop adding water.
Stir in 24 ounces tomato puree and 1/2 cup coarsely chopped pitted Kalamata olives and bring to a simmer, about 2-3 minutes.
Stir in the meatballs and any accumulated juices from the plate. Nestle the meatballs into the tomato sauce, and simmer for about 5-10 minutes, until the meatballs are cooked through and the tomato sauce has thickened slightly.
Remove the skillet from the heat and stir in 2 Tablespoons chopped parsley. Sprinkle with 1/4 cup crumbled feta cheese, and up to another Tablespoon of chopped parsley if desired. Serve with pasta, cooked grains, or roasted potatoes and salad to make a complete meal.
Make-Ahead Instructions
You can make this recipe ahead of time to a few different degrees:Least Made-Ahead: Complete the meatball portion of the recipe through step 2. Keep the rolled meatballs covered in the fridge until ready to proceed with the recipe.More Made-Ahead: Complete the meatball portion of the recipe through step 4. Keep the cooked meatballs in the fridge until you're ready to proceed with the sauce portion of the recipe (note that this method may require you to simmer the meatballs in the sauce a little longer to ensure they're fully warmed/cooked thorugh).Most Made-Ahead: Complete the all of the steps in the meatball portion of the recipe and through step 3 of the sauce portion. Place both the sauce and the cooked meatballs in the fridge until ready to complete the recipe. When you're ready to make dinner, heat the sauce in the skillet or in a saucepan over medium heat until it reaches a simmer. Add the meatballs and simmer for 10-15 minutes until fully cooked/heated through.
Notes
The size of cans/jars of tomato puree that are available can vary. I use a 670mL (23oz) jar, but it's also often available in 15 or 28oz jars. If you have anywhere in the realm of 23-28 ounces, just use that - don't worry about measuring out a specific amount. Note that there is a difference between tomato puree and tomato sauce. Tomato sauce is often thinner than tomato puree and has more seasonings added to it. I recommend using crushed tomatoes instead of tomato sauce if you can't find tomato puree.