In a large pot over medium heat, add the oil and mustard seeds. Heat, stirring occasionally, until they start to pop (and turn a little brown), then add the garlic and yellow onion. Sauté, stirring occasionally, another 2-3 minutes or until the onion is soft and translucent.
Next, add the soy sauce/tamari, tomatoes, Spanish paprika and collard greens. Stir well. Cover, then reduce the heat to medium/low and let simmer for 20 minutes, stirring every 5 minutes or so.
Add the salt and pepper, and taste to adjust seasonings (add additional salt, and/or nutritional yeast, cayenne pepper, etc.). Serve.