Heat the oil in a large skillet over medium heat. Add the onion, cumin, chili powder, and garlic and cook until you can smell the spices and the onion is softened, about 3 minutes. Stir in the tomatoes with the 1/3 cup of reserved juice and the tomato paste, then stir in the beans. Season with salt and pepper to taste. Let the bean mixture simmer until everything is hot, about 3 minutes. Add the corn and spinach and stir until the spinach has wilted and everything is well blended and hot, about 3 minutes. Taste for seasonings, adding more salt and/or pepper as necessary.