In a pot over medium heat, heat oil. Add onions, garlic, and ginger and cook until softened.
Add tomatoes and cook, mashing tomatoes with the back of the spoon, until softened and release juices.
Add fish sauce and cook for about 30 to 40 seconds.
Add water and bring to a boil. Add mussels and cook, skimming scum that floats on top, for about 3 to 5 minutes.
Season with salt and pepper to taste.
Add spinach. Turn off and allow spinach to cook in residual steam. Serve hot.
Notes
Remove the beard or byssus when you're ready to cook the mussels, as they will begin to spoil once it's pulled off. Keep them in a bowl of cold water and refrigerate until set to use.