Place the chorizo in a medium sauté pan over medium-low heat and cook, stirring until its oil releases, about 4 minutes. Add the onion and garlic and cook, stirring, until softened and caramelized. Add the sachet, white wine, and vinegar and simmer until almost dry. Add the heavy cream and stock and simmer lightly for 20 minutes. Remove the sachet and transfer to a blender; puree until smooth. Pass through a fine-meshed sieve into a small bowl and, if needed, add extra chicken stock to adjust to a consistency that will lightly coat a spoon. Store, chilled.