In a large skillet, heat the olive oil over medium heat. Add the onions, celery and garlic and cook, stirring often, until softened, 7 minutes. Transfer 1 1/3 cups of the onion mixture to a large bowl; let cool. Reserve the skillet.
Using a food processor, puree the tomatoes and roasted peppers. Add the puree to the skillet and bring to a boil over medium heat. Stir in the paprika and season with salt and pepper; transfer to a large slow cooker, cover and set to low temperature.
Whisk the breadcrumbs, egg, 1/2 teaspoon salt and 1/4 teaspoon pepper into the remaining onion mixture. Mix in the turkey. Form 1 1/2-inch meatballs. Add the meatballs to the slow cooker. Cover and cook on low heat for 6 hours.