Preheat the oven to 400°F.
Crack the eggs and egg whites into a large bowl. Add 1/4 teaspoon of the salt and a pinch of fresh cracked pepper and beat until blended. Mix in the Gruyere cheese if using.
Heat a 10-inch nonstick oven-safe skillet over medium heat.
Add the olive oil and onions to the pan, cooking until the onions are golden, about 5 to 6 minutes.
Next, add the sweet potatoes and thyme, seasoning with 3/4 teaspoon of salt, garlic powder, paprika, and a pinch of black pepper. Cover and cook over medium-low heat, stirring occasionally, until the sweet potatoes are crisp and tender, around 12 minutes.
Stir in the chopped turkey to the skillet, ensuring it's well combined with the veggies. Then, add the spinach and cook until it wilts, about 2 to 3 minutes.
Pour the egg mixture into the skillet, spreading it evenly. Reduce the heat to low and cook until the edges are set, about 6 to 8 minutes.
Transfer the skillet to the oven and bake until the frittata is completely set and cooked through, approximately 8 to 10 minutes.
Remove from the oven and let it cool slightly before slicing it into 6 wedges. Serve hot.
