To make the tomato-sausage base, put a saucepan over medium-high heat and add the oil. Add the onions and cook until translucent, stirring, about 2 to 3 minutes. Add the chorizo and saute for 1 to 2 minutes. Add the tomatoes and wine, reduce the heat to a simmer, and simmer gently until slightly thickened and reduced, about 10 to 15 minutes. Season, to taste, with salt and pepper. Keep the tomato base warm while you cook the mussels.
Put the mussels in a large skillet or wok over high heat, add the stock or wine, and cover with a lid. Cook the mussels until they open, about 5 minutes. Remove the lid, and pour the tomato-sausage base over the mussels. Cook for 1 to 2 minutes, until fully heated through. Divide the mussels among 4 shallow soup bowls and serve immediately, garnished with lemon wedges.
*Cook's Note: At the Dahlia Lounge, we serve these mussels with lemon aioli, which is homemade mayonnaise seasoned with plenty of lemon juice and lemon zest. We also make our own spicy pork sausage, but Spanish chorizo is a good substitute. Another way to cook the mussels is to roast them in a very hot oven, with a lid on, until they open.