Heat the olive oil in a wide, deep skillet over medium heat. Add the onion and cook for 5 to 6 minutes, until softened.
Add the garlic and cook for 30 seconds until fragrant.
Stir in the tomato paste, smoked paprika, and cumin, and cook for 1 minute, until darkened slightly.
Pour in the crushed tomatoes and vegetable broth. Season with salt and pepper and simmer for 10 minutes, stirring occasionally, until slightly thickened.
Discard any mussels that are already open and don't close when tapped.
Nestle the mussels into the simmering sauce, cover the pan, and cook for 5 to 7 minutes, listening for the shells to open.
Once the clicking has mostly stopped, remove the lid and discard any mussels that remain closed.
While the shakshuka finishes, brush warm flatbread with olive oil and sprinkle generously with za'atar.
Garnish the finished shakshuka with fresh parsley and serve directly from the pan with the za'atar flatbread alongside.