Heat the olive oil in a large pan over medium-high heat. Add the diced bell pepper and halved cherry tomatoes, sauteing for 4 to 5 minutes until the tomatoes become blistered and saucy.
Stir in the minced garlic and cook for about 1 minute more.
Add the baby spinach and the white parts of the green onions to the pan. Stir for 2 to 3 minutes until the spinach is fully wilted, then transfer all the vegetables to a plate.
Add the ground beef, dried oregano, salt, and black pepper to the empty pan. Break up the beef with a spatula and cook until thoroughly browned, then drain off any excess grease.
Return the cooked vegetables to the pan and stir in the green parts of the green onions, cooking until everything is heated through.
Garnish with crumbled feta if desired, and serve.