Toss steak strips in a large zipper-topped plastic bag with oregano, garlic, salt and pepper; seal bag and refrigerate overnight or for at least 1 hour.
Heat half of the olive oil in a large skillet over medium heat; add steak strips and cook, stirring often, for 5 to 7 minutes or until they reach desired level of doneness; remove from pan. Add broth to the skillet and whisk up all of the browned bits from the bottom of the pan.
Add remaining olive oil and, when hot, add onion, bell pepper and mushrooms; cook for 5 to 7 minutes or until veggies are tender-crisp.
Return steak strips to the skillet and heat through. Remove skillet from the heat and place cheese slices on top of steak/veggie mixture; cover and allow skillet to sit for 5 minutes or until cheese has melted.
Serve over spinach and enjoy!