Remove the core from the green cabbage with a sharp knife and slice it into thin strips.
Set aside about half of the chopped cabbage for later layering.
In a large skillet over medium heat, add olive oil and diced onions and cook until translucent (about 5 minutes).
Add minced garlic to the skillet and sauté for another minute until fragrant.
In a large mixing bowl, combine cooked ground turkey, sautéed onions and garlic, cooked rice, diced tomatoes with their juice, dried herbs, and pepper flakes if using.
Mix well until everything is evenly distributed.
In the slow cooker, layer half of the chopped cabbage at the bottom as a base layer.
Spoon half of the turkey mixture on top of that cabbage layer.
Repeat with another layer of cabbage followed by the remaining turkey mixture.
Pour enough chicken broth to cover everything (about 1-2 cups depending on desired sauciness).
Cover the slow cooker with its lid and set it on low heat for 6-8 hours or high heat for about 4 hours.
When cooking is complete, gently stir everything before serving.
Garnish with fresh parsley or grated cheese if desired.