Add white wine and turn heat down to medium high, stirring until it mostly evaporates, about 1-2 minutes. Add chicken stock, tomatoes, browned chorizo, and fish sauce and bring to a simmer. Once simmering, taste for salt. Surprisingly, I didn't need to add any salt ( salt content in stocks and chorizo can vary greatly - so make sure to taste. Add cracked pepper. Add fish, simmer a couple minutes and add prawns, simmer a couple minutes, then add mussels. Remember the larger the prawns or mussels or fish pieces the longer they take to cook, so look at all your seafood ingredients and determine which will take the longest to cook, putting them in first.
In same skillet, heat 2 T olive oil on med high heat. Add fennel, stirring often for about 3 minutes. Add onion, turn heat down to med and saute both until tender, about 8-10 minutes. Add garlic, sauté 3 minutes, stirring occasionally, until garlic starts turn golden. Add tomato paste. Turn heat up to high, constantly stirring, until paste darkens, about 3 more minutes. You are basically frying the paste to deepen the flavor of the dish.
In a large heavy bottom deep skillet or Dutch oven, brown 6-8 ounces chorizo in a little olive oil. (Remove from casing and break up or crumble into small bite size pieces). Once browned, set aside. Pour off the fat, wipe out skillet with paper towel.