In a large (12-inch) skillet, heat the oil over medium-high.
Add the tempeh and onion, season with salt and pepper, and cook, undisturbed, until deep brown, 2 minutes. Stir and cook until browned all over, 3 to 5 minutes. Add more oil as needed if the pan looks dry.
Add the tomato paste and stir in well. Then add the cumin, chili powder, nutritional yeast, smoked paprika, a bit more salt. Cook, stirring constantly, until brick red and beginning to stick to the skillet, 5 to 7 minutes. Try not to rush this step; if it begins to burn, deglaze with a bit of water and reduce the heat.
Add 1 cup water and the hot sauce, if using. Cook, stirring and scraping up browned bits, until thickened and the mixture is sizzling, 2 to 4 minutes. Season to taste with salt and hot sauce.
For tacos, divide the filling between the taco shells, then top with chosen toppings.
OR, make a taco salad by breaking up a few tortilla chips and stirring with the filling.
Toppings you might try: grated cheese, lime, shredded lettuce, chopped onion, pico de gallo, sour cream, cashew crema, sliced jalapeños, cilantro, avocado, hot sauce.