Place a non-stick skillet over medium-high heat.
Add the ground beef to the skillet, breaking it apart with a wooden spoon or spatula.
Season the beef with the dried oregano, sea salt, and black pepper.
Cook the beef until it is thoroughly browned and cooked through, approximately 7 to 8 minutes, then remove from the heat.
While the beef cooks, halve the cherry tomatoes and thinly slice the cucumber.
In a large salad bowl, combine the mixed baby greens, cherry tomatoes, and cucumber.
Whisk together the lemon juice and extra virgin olive oil in a small bowl to create a light dressing.
Drizzle the dressing over the vegetables and toss gently to coat.
Top the dressed greens with the warm sautéed ground beef and sprinkle the crumbled feta cheese over the top before serving.