Heat the oil in large nonstick skillet over medium heat. Add the onion, chile, and garlic. Saute until softened, about 2 minute Add the tomato and cook, stirring, until mixture appears fairly dry, about 3 minute Stir in the mushrooms and salt, cook, covered, stirring occasionally, until mushrooms are tender but not mushy. Stir in the sour cream and heat through.
Meanwhile, heat the taco or tostada shells according to pkg directions. Keep warm.
To serve, spoon 1/4 cup of mixture onto each shell, sprinkle with cilantro.