In a 10” cast iron skillet over medium-low heat, warm the oil. Add the onions, cover the pan, and cook for 5 minutes. Add the garlic and cook for 2 minutes, until slightly browned. Increase the heat to medium-high and add the chard and kale stems; cook for two minutes. Add the chard and kale leaves and ¼ cup of water, partially cover and cook just until wilted, about 30 seconds. Add the cumin, paprika, salt, pepper, and tomatoes and bring to a boil. Cook for 2 minutes, then add the spinach and stir until wilted.