Pour the liquid from the foil-baked salmon into a saucepan.

Add the chopped or sliced vegetables that were baked with the salmon to the saucepan.

Bring the mixture to a boil, then reduce heat and simmer for about 15 minutes, or until reduced to 1 1/2 cups.

Strain the sauce to remove the vegetables, and return the strained liquid to the saucepan.

Add the heavy cream to the strained sauce.

Prepare a beurre manie by blending the softened butter and flour until a smooth paste forms.

Bring the sauce to a simmer and gradually whisk in the beurre manie until smooth.

When the sauce is thick and smooth, add the cold butter bit by bit, stirring constantly until melted and incorporated.

Stir in the lemon juice, salt, and cayenne pepper to taste.

Remove the sauce from the heat.

Serve the sauce hot over the cooked salmon.