1. Prepare the Spinach:
* If using fresh baby spinach, finely chop it. You can quickly wilt it by microwaving for 30-60 seconds or by sautéing it in a dry pan for a minute until it just wilts. Squeeze out any excess moisture thoroughly. This step is crucial to prevent the meatballs from becoming too wet. If using frozen chopped spinach, thaw it completely and squeeze out all excess water. You’ll need about 1/2 cup of thawed, squeezed spinach.

2. Make the Meatball Mixture:
* In a large mixing bowl, combine the ground chicken, chopped and squeezed spinach, panko breadcrumbs, grated Parmesan cheese, beaten egg, onion powder, garlic powder, Italian seasoning, 3/4 teaspoon salt, and 1/2 teaspoon black pepper.
* Gently mix with your hands until just combined. Be careful not to overmix, as this can make the meatballs tough. The mixture should be cohesive but still light.

3. Form the Meatballs:
* Lightly wet your hands with water to prevent sticking.
* Roll the mixture into meatballs, about 1.5 inches in diameter. You should get approximately 20-24 meatballs. Place them on a clean baking sheet or plate.

4. Optional Browning Step (Recommended for Best Flavor and Texture):
* Heat 1-2 tablespoons of olive oil in a large skillet over medium-high heat.
* Carefully place the meatballs in the hot skillet, working in batches if necessary to avoid overcrowding.
* Brown the meatballs on all sides, about 1-2 minutes per side. They don’t need to be cooked through, just nicely browned. This step adds a fantastic layer of flavor and helps them hold their shape better in the slow cooker.
* Remove browned meatballs and set them aside. If you’re skipping this step, you can place the raw meatballs directly into the slow cooker, but they might be slightly more delicate.

5. Prepare the Sauce in the Slow Cooker:
* To the basin of your slow cooker (minimum 4-quart size, 6-quart is ideal), add the crushed tomatoes, tomato paste, chopped yellow onion, minced garlic, dried oregano, dried basil (if using dried), red pepper flakes (if using), sugar (if using), 1/2 teaspoon salt, and 1/4 teaspoon black pepper.
* Stir well to combine all the sauce ingredients. If the sauce appears very thick, stir in the 1/2 cup of water or chicken broth.

6. Add Meatballs to the Slow Cooker:
* Gently nestle the (browned or raw) meatballs into the sauce in the slow cooker. Try to arrange them in a single layer as much as possible, but it’s okay if they overlap slightly. Spoon a little sauce over the tops of any exposed meatballs.

7. Slow Cook to Perfection:
* Cover the slow cooker and cook on LOW for 4-6 hours or on HIGH for 2-3 hours.
* The meatballs are done when they are cooked through (internal temperature of 165°F / 74°C) and the sauce is bubbly and fragrant. Avoid overcooking, especially on high, as ground chicken can dry out if cooked for too long.

8. Final Touches (Optional):
* If using fresh basil, stir it into the sauce during the last 15-20 minutes of cooking or just before serving for the brightest flavor.
* Taste the sauce and adjust seasonings (salt, pepper, sugar) if needed.