Step 1

Combine ground beef, 1/2 cup onion, panko, eggs, broth, parsley, 1 teaspoon garlic, fennel and 1/2 teaspoon each salt and pepper in a large bowl. Mix with your hands until well combined (do not overmix).

Step 2

Scoop about 2 tablespoons of the beef mixture into the palm of your hand and flatten; top with a mozzarella cube. Wrap the mozzarella entirely in the beef mixture to seal; shape into a 2-inch ball. Place on a plate or parchment-paper-lined baking sheet. Repeat with the remaining beef mixture and mozzarella to make 24 meatballs; refrigerate, uncovered, for 30 minutes.

Step 3

Heat oil in a large high-sided nonstick skillet over medium-high heat. Add the meatballs in batches if necessary; cook, occasionally turning gently to brown all sides, for 4 to 6 minutes. Transfer to a plate (do not wipe out the pan).

Step 4

Add the remaining 1/2 cup onion and 2 teaspoons garlic to the pan; cook over medium-high heat, stirring occasionally, until softened, 2 to 3 minutes. Add crushed tomatoes, diced tomatoes, thyme, basil and the remaining 1/4 teaspoon each salt and pepper. Stir well and bring to a simmer over medium-high heat. Transfer the meatballs and any juices from the plate to the pan, stirring gently to coat. Reduce heat to medium-low, cover and simmer until a thermometer inserted in the center of the meatballs registers 160°F, about 15 minutes.

Step 5

Serve the meatballs with the sauce; garnish with Parmesan and basil, if desired.