1 Rinse tomatoes, wipe dry and cut out stems. Halve tomatoes and remove seeds. Roast tomatoes under preheated broiler until blackened and blistered, watching carefully. Allow tomatoes to cool slightly, then remove skins.
2 Peel onions and garlic and chop both finely. Heat 1 tablespoon oil in a pot. Sauté onions and garlic until translucent over medium heat.
3 Add tomatoes, chili powder and cumin and sauté briefly, then pour in the chicken stock. Cook for 30 minutes over medium heat.
4 Meanwhile, rinse mussels thoroughly, discarding any that remain open, then drain. Remove beards from mussels and rinse again.
5 Heat remaining oil in another pot, add mussels and cook briefly. Pour in wine, cover and cook until mussels have opened, 3-4 minutes. Discard any mussels that do not open.
6 Remove mussels with a skimmer from the pot, then strain cooking liquid through a sieve.
7 Rinse scallions, pat dry and cut into thin rings. Season yogurt with salt in a bowl and stir until smooth.
8 Squeeze juice from lime. Crush taco shells in a freezer bag and add to tomato mixture in pot.
9 Cook taco shells in the soup for 4 minutes. Puree until smooth with an immersion blender and season with salt, pepper and 2 tablespoons lime juice.
10 Add mussels to the soup. Sprinkle witih scallions and serve immediately with yogurt.