In a large, heavy stockpot, heat the olive oil.
Add the onion, garlic and jalapeño and cook over moderately high heat, stirring, until softened, about 3 minutes.
Add the tomato, oregano and saffron and cook for 1 minute, stirring.
Sprinkle the flour over the vegetables and stir it in. Add the wine and mussels, cover the pan and cook, stirring, for 1 minute.
Add the fish stock, parsley and lemon juice and cook over high heat, stirring, until the mussels open, about 5 minutes. Discard any mussels that do not open.
Transfer the mussels and their sauce to deep bowls. Serve at once with crusty bread to soak up the sauce.