Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Combine the turkey, breadcrumbs, Parmesan, pesto, salt, pepper and egg in a large mixing bowl with your hands, taking care to not overwork the meat. Form golf ball-size meatballs (about 24) and stuff each with 1 mozzarella cube. Evenly space the meatballs on the baking sheet. Bake until just starting to brown, 15 minutes.
Add the Classic Tomato Basil Sauce and chicken broth to the slow cooker. Transfer the meatballs to slow cooker, cover and cook on low 4 to 6 hours.
Allow the meatballs to cool slightly before serving, and adjust with salt and pepper, if necessary.
Cut an "X" in the bottom of each tomato and blanch in a large pot of boiling water 10 seconds. Immediately transfer the tomatoes with a slotted spoon to an ice bath to cool, then peel, seed and chop.
Cook the garlic in the oil in a small, heavy pot over medium heat, stirring occasionally until golden, 3 to 5 minutes. Add the tomatoes and red pepper flakes, season with salt and pepper, and add the fresh basil. Cover and simmer 1 hour, stirring occasionally.