Heat the oil in a large skillet over medium-high heat. Brown the beef and onion for 7 to 8 minutes, breaking up the meat and cooking it to 160°F.
Add the garlic and stir for 30 seconds. Fold in the beans, broth, oregano, salt and pepper.
Simmer over medium heat for 4 to 5 minutes, stirring gently, until the beans are hot and most of the broth has reduced.
Add spinach in two handfuls and turn it through the filling for 2 minutes until wilted.
Remove from the heat, stir in lemon juice and scatter feta over the skillet. Serve in shallow bowls while the beans are hot.