In a bowl, season the thinly sliced beef with salt, pepper, oregano, cumin, and smoked paprika. Let it marinate for about 15 minutes.
In a large frying pan or wok, heat the olive oil over medium-high heat.
Add the marinated beef to the pan in a single layer. Sear it for about 3-4 minutes until browned but not fully cooked. Remove the beef from the pan and set aside.
In the same pan, add the sliced onion and bell peppers. Stir-fry them for about 5 minutes until they begin to soften. Add minced garlic and cherry tomatoes, cooking for another 2-3 minutes until fragrant and the tomatoes start to blister.
Return the beef to the pan and add the fresh spinach. Cook for another 2-3 minutes, stirring continuously until the beef is cooked through and the spinach is wilted.
Drizzle lemon juice over the stir-fry, give it a final toss to combine the flavors. Season with salt and pepper to taste.
Transfer the Mediterranean Beef Stir Fry to a serving dish and sprinkle with feta cheese if desired.