Fish Mousseline with Sauce Nantua

Name: Fish Mousseline with Sauce Nantua
Ingredients: 2 tablespoons butter; 1 carrot, finely chopped; 1 onion or 2 shallots, very finely sliced; 8 ounces/250 g raw shrimp or crayfish with shells and heads on; 2 tablespoons Cognac; 1/2 cup/125 ml dry white wine; 1 1/2 cups/375 ml fish stock; 2 teaspoons tomato paste; Pinch cayenne pepper; Pinch paprika; 1 bay leaf; 1 fresh thyme sprig; 1 1/4 cups/300 ml heavy cream; Salt and pepper; 12 ounces/330 g pike fillets, scallops or other light fish like sole or haddock, chilled; 1 cup/250 heavy cream, chilled; 1 egg; 2 generous handfuls finely chopped fresh parsley or tarragon