To make the sauce, working in batches, combine the marinara sauce, tomatoes and red bell peppers in a blender and puree. Set aside.

In a food processor, finely dice the garlic and shallots.

In a large pot over high heat, warm the olive oil. Add the garlic and shallots and cook until translucent, stirring constantly so they don't brown. 

Add the tomato–bell pepper puree, the sweet potato tomato sauce, salt, sugar, coriander, cumin and red pepper flakes and stir well. Reduce the heat to low and simmer, stirring occasionally, for 45 minutes while you make the meatballs.

To make the meatballs, in a large pot over medium-high heat, bring the stock to a simmer.

Meanwhile, in a large bowl, combine the ground beef, sausage, onions, carrots, rice, salt, lemon zest, garlic and parsley and mix gently with your hands just until the ingredients are well blended. Roll the meat mixture into balls and place on a large plate.

Transfer the meatballs to the simmering stock, reduce the heat to medium and simmer until the meatballs are cooked through, 25 to 30 minutes.

To finish, using a slotted spoon, transfer the meatballs to the pot with the sauce. Add a small ladleful of the stock to the pot and stir, adding more stock as needed to thin the sauce to the desired consistency. Heat for 10 minutes, then serve the meatballs with the sauce.