Preheat oven to 375°F.
Pour oil into a heavy bottom skillet, over medium-low heat. Add all soffrito ingredients into skillet, except tomatoes, and stir. Season with salt and pepper. Allow mixture to caramelize for about 1 ½ hours, stirring occasionally.
Stir in tomatoes and cook down an additional 30 minutes or until the mixture becomes thick. Season with salt and pepper. Remove from heat and allow soffrito to cool completely.
Pour stock and saffron into a medium saucepan, place over medium heat and bring to a simmer. Poach seafood in the saffron infused stock, one type at a time, until shrimp and octopus have just cooked through and the clams and mussels have just opened. Remove from stock and set aside covered.
Pour oil into a 14-16” paella pan (or heavy bottom skillet) and place over medium-high heat. Brown chorizo for about 1 to 2 minutes and remove from pan. Set aside.
Drain all but 2 tablespoons grease from pan and add fideua, chorizo and ¼ cup soffrito to pan and season with salt and pepper. Stir together and sauté for about 2 minutes.
Pour hot stock over fideua mixture and bring to a boil. Once mixture has come to a boil add octopus back into pan (gently tucking each piece down into fideua mixture). Lower heat to medium-low and simmer for 20 to 25 minutes or until all the liquid has evaporated and fideua has cooked through. Uncover, raise heat to high and cook for 3 to 4 minutes to create socarrat, or crispy, golden crust.
Remove from heat and top with remaining poached seafood (slightly warm seafood, if needed). Finish with a sprinkle of tarragon and lemon wedges. Serve.
