Combine garlic, vinegar, 1/2 teaspoon salt, oregano, pepper and 1 teaspoon olive oil in gallon-size resealable plastic food-storage bag. Add steak; seal. Refrigerate to marinate at least 1 hour.
Remove beef from marinade; drain well. Discard marinade. Heat 1 teaspoon oil in large nonstick skillet over high heat. Add beef; sauté until cooked through, about 5 minutes. Remove beef from skillet to plate; cover to keep warm.
Add remaining teaspoon oil, onions and mushrooms to skillet; sauté until onions are softened and mushrooms give off their juices, about 6 minutes. Add tomatoes, jalapeno and beef; cook until heated through, about 2 minutes.
Stir together cornstarch, remaining 1/2 teaspoon salt and the water in small bowl until smooth. Pour into skillet; cook, stirring constantly, until mixture thickens and bubbles, about 2 minutes.
Spoon filling into warmed tortillas. Wrap up tortillas and serve.
