Start by getting your ingredients ready. Dice the onion.
*Make the roux (heat vegetable oil on medium/high, add flour, stir continuously until oil flour mixture browns). The amount of oil is about equal to the amount of roux required by the recipe. For instance, the original recipe calls for 2 oz of roux. So 4 tbsp oil and 4 tbsp white flour. Set aside.
Melt Butter on medium heat. Add diced onion and garlic, stirring frequently, about 5 minutes.
Add white wine. Turn up heat to high until boiling then back down to medium. Cook 5 minutes on medium.
Stir in roux, shrimp stock, and Old Bay Seasoning. Cook 3 minutes.
Add milk and heavy cream. Turn heat on high just until mixture starts to boil. Turn to medium/low, cover, and stir occasionally for 5 minutes. The sauce is now ready! It can be store for up to one week in the regrigerator.
Heat oil in saute pan over medium flame. Put mushrooms inches Cook 2 minutes.
Add mussels, and cook 30 seconds.
Add shrimp, crawfish, and scallops. Cook until heated through, stirring.
Add portofino sauce; cook until bubbling throughout.
Move mushrooms and seafood to one side of pan. Add linguine to other side. Using tongs,coat pasta with sauce. (mussels should be open).