Heat the oil in a sauté pan over medium high heat or use a dry nonstick skillet. Add the garlic and sauté for a minute but don’t let it brown. Add the onion and sauté it for 5 minutes or until it’s translucent, adding a tablespoon or two of liquid to prevent sticking if necessary. Add the cooked beans and the tomato and cook for another few minutes until the tomato is incorporated. Add the lime juice and spinach. Cook until the spinach is wilted. Add salt, to taste. Serve hot.