Heat 1 tablespoon of the olive oil in a large soup pot or Dutch oven over medium heat. Sauté the onion for about 5 minutes, until fragrant and softened. Remove from the heat, transfer onions to a bowl and let cool a few minutes. Using your hands, combine the onion with the ground beef, garlic salt, black pepper, feta cheese, 1/2 of the parsley and egg. Form into 1-inch meatballs.
Heat 2 tablespoons olive oil in the pan over medium-high heat. Add the meatballs and cook about 4 minutes, turning a few times so all sides brown. Transfer the meatballs to a plate. Add the carrots and celery to the pot cook until just softened, about 5 minutes. Add the broth and bring to a boil. Add the meatballs into the broth along with the other 1/2 of the parsley. Simmer until the meatballs are cooked thorough about 2-3 minutes.
Add the tortellini and cook until they float. Add the spinach and cook until it wilts, about a minute. Serve while hot.
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