Cut each pita pocket in half and set the 4 halves aside. Tear the romaine leaves into pieces roughly the size of the pita openings and set aside.

Heat a large skillet over medium heat. Add 1 tablespoon olive oil.

Add the 4 ounces sliced white mushrooms to the skillet and sauté, stirring occasionally, about 6 minutes, until the mushrooms are softened and lightly browned.

Add the sliced red onion, minced garlic, and red bell pepper strips to the skillet. Continue to sauté, stirring occasionally, until the onion and pepper are tender, about 4 minutes.

Add the 1 pound beef sirloin tip strips to the skillet. Cook on medium heat, stirring occasionally, 5–10 minutes, until the beef is no longer pink and cooked through.

Sprinkle the cooked mixture with salt to taste, the 1/2 teaspoon black pepper, and the 2 teaspoons dried oregano. Stir well to combine.

Reduce heat to low, cover the skillet, and simmer for 5 minutes to meld the flavors.

After simmering, remove the lid and use a slotted spoon (or transfer the mixture to a colander briefly) to drain off and leave any excess liquid in the pan.

Open each pita half, place a torn romaine leaf or leaves inside, divide the meat-and-vegetable mixture evenly among the 4 pita halves, and top each with a dollop of the 1/4 cup plain nonfat Greek yogurt (divide the yogurt among the sandwiches).