Melt butter in a large skillet. Whisk in cream, ketchup and spices. Bring to a boil. Continue to simmer and reduce while you continue with the rest of the recipe, stirring occasionally with a wire whisk. (most of the cream will boil away, leaving you with a dark orange sauce).
Sauté mushrooms in a separate small skillet over medium to low heat, in olive oil until soft. Add mushrooms to simmering sauce.
Cook shrimp (and crawfish if available) in the small skillet in olive oil just until pink and add to sauce.
Simmer for about 5 minutes adding green onions for the last 2 minutes or so.
In a large bowl, toss shrimp mixture with cooked and drained fettuccine and serve immediately, topping with finely shredded fresh parmesan (or romano) cheese.