Heat a large skillet over medium heat. Add the ground turkey and cook, breaking it apart with a wooden spoon or spatula, until no pink remains. Transfer to a plate and set aside.
Sauté the onion. Add the olive oil to the same skillet. Add the onion and cook until it starts to turn translucent.
Add the vegetables. Add the mushrooms and jalapeño. Once the mushrooms start releasing their moisture, add the red bell pepper. Cook until the vegetables begin to soften.
Add the garlic. Stir in the minced garlic and cook briefly, just until fragrant.
Add the cabbage. Add the sliced cabbage and toss it with the vegetables. Cover and cook for a few minutes. Remove the lid, toss again, then cover and continue cooking until the cabbage is tender but still has some texture.
Return the cooked turkey to the skillet. Add the garlic powder, salt, black pepper, and sumac. Toss everything together until well combined and heated through.
Stir in most of the scallions and the parsley, if using. Taste and add more salt or sumac if needed.
Top with the remaining scallions and another sprinkle of sumac before serving.
