Steak Fajita Sandwiches

Name: Steak Fajita Sandwiches
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: Mexican, TexMex
Ingredients: 4 ounces white mushrooms (sliced); 1 tablespoon olive oil; 1 red onion (medium, sliced into strips); 2 garlic cloves (minced); 1 red bell pepper (medium, cut into strips); 1 pound beef sirloin tip steak (cut into thin strips - grass-fed beef recommended); salt to taste; 1/2 teaspoon black pepper; 2 teaspoons dried oregano; 2 pita pockets (cut in half - gluten free or whole-wheat); 4 romaine lettuce leaves (torn); 1/4 cup Greek yogurt (plain nonfat)