Heat the oil in a large pan on medium-high heat. Add the bell pepper and cherry tomatoes and saute for 4-5 minutes, until the tomatoes are blistered, and saucy. Add the garlic and stir another minute.
Add the spinach and white parts of the green onion. Note that the spinach will take up most of the pan, but it will wilt down. Stir for another 2-3 minutes, until the spinach has wilted. Remove these veggies to a plate.
Add the ground beef, oregano, salt, and pepper to the pan, and use a spatula to break up the meat. Cook the ground beef until it’s browned, then drain off any excess grease.
Transfer the veggies back to the pan, add the green parts of the green onion, and stir until warmed through. Garnish with crumbled feta before serving.