Place tofu, soy sauce, nutritional yeast, cornstarch and turmeric (if using) into the bowl of a food processor fitted with an S-blade. Process until smooth and creamy, stopping to scrape down sides of bowl as needed.
Coat a medium skillet with olive oil and place over medium heat. Add onion and potato. Sauté until lightly browned in spots, about 7 minutes. Remove from heat. Add garlic, scallions, bell pepper, spinach and black pepper. Fold in tofu mixture until evenly mixed.
Preheat oven to 375°F. Lightly oil 9 inch by 9 inch baking dish and transfer tofu and veggie mixture to dish. (If you're using an oven-safe skillet, you can skip this step.) Smooth out the top with a spatula.
Bake until firm in the center and lightly browned, about 35 minutes. Remove from oven and allow to sit 5-10 minutes before cutting.
Dice your veggies finely to ensure everything mixes well and your frittata holds together when cut. You want between ¼ and ½ inch dice for the potato and pepper.
If you have a 10 inch oven proof skillet you can use it for both sautéing and baking, avoiding having to transfer your mixture and dirty an extra dish.