For the sauce:
For the meatballs:
To make the sauce, add the tomatoes to a blender or food processor. Puree until smooth, adding a bit of the reserved liquid if necessary to smooth the mixture out. Melt the butter in a large, deep skillet or sauté pan over medium-high heat. Add the onion and cook about 1 minute, until it begins to soften. Stir in the tomato paste, garlic and red pepper flakes and mix just until fragrant, about 30 seconds. Stir in the tomato mixture, lower the heat to a simmer, and cook about 5-8 minutes, until the sauce is slightly thickened. Season to taste with salt and pepper. Remove the pan from the heat and stir in the cream. Set aside.
To make the meatballs, combine the panko, grated onion, parsley, basil, Parmesan, salt, pepper, garlic and egg. Stir together with a fork just until blended. Mix in the ground chicken and knead together gently until evenly combined. Form the mixture into meatballs about 1½-inch in diameter.
Preheat the oven to 400˚ F. Heat the olive oil in a large skillet or Dutch oven over medium-high heat. Add the meatballs to the pan in a single layer (two batches may be necessary if you don’t have enough surface area.) Let cook, turning occasionally, until all sides are browned.**
Once all of the meatballs are browned, place them in the pan with the tomato sauce. Sprinkle the mozzarella and additional Parmesan over the top. Bake until the cheese is fully melted and bubbling and the meatballs are cooked through, about 15 minutes. Remove from the oven and garnish with the fresh basil. Serve warm.
*For best results, always use freshly shredded cheese. Pre-shredded cheese comes coated in things such as flour or cornstarch to prevent clumping and results in an unpleasant, gritty texture when melted.
**I added this browning step because we prefer the flavor and texture it adds, but if you prefer, you can skip the step of browning the meatballs entirely. Just add the shaped meatballs to the tomato sauce and bake until they are cooked through. You will probably need to increase the baking time.