Heat two tablespoons of olive oil in a deep skillet or ductch oven over medium heat.
Add the fennel, onion, shallots, and salt. Saute until the onion becomes translucent about 5-7 minutes.
Add the garlic and crushed red pepper and continue to saute for 2 minutes.
Add the tomato paste, plum tomatoes and all juices, wine, chicken stock, and bay leaf.
Bring the sauce to a light boil, then reduce the heat to simmer and allow to cook for one hour, stirring occasionally
While the sauce is simmering using another saute pan, add 2 tablespoons olive oil and sear the shrimp, scallops (and fish pieces if used) on both sides, making sure not to fully cook them (about half way is fine)
Remove the seafood from the pan and place it on a plate until needed. If you did use the fish pieces you can add them to the sauce now. *do not add the shrimp or scallops at this time In the same pan add a little more oil, the clams and mussels. Cover and steam them until they open. *If any of the mussels or clams are open before cooking discard them, they're dead and aren't safe for consumption.
At about the 45-minute mark of simmering the sauce, add in the crabs, mussels and clams with all the pan juices. Continue to simmer.
If you are serving your Choppino over pasta you may begin getting the water ready at this time.
With five minutes of cooking time remaining, add in the shrimp and scallops.
Taste the sauce and re-season as needed.
If you cooked pasta serve over pasta. If not, enjoy this delicious stew with a loaf of crusty bread.