Sauté aromatics: In a medium skillet, melt butter over medium heat. Add shallot and garlic, sauté for 2–3 minutes until fragrant and soft.
Add sherry: Pour in the sherry and let it simmer for 2–3 minutes to reduce slightly.
Stir in cream: Lower the heat, then slowly add the cream, whisking continuously.
Season & thicken: Add Dijon mustard (if using), salt, and pepper. Simmer gently for 5 minutes until the sauce thickens to your liking.
Finish & serve: Remove from heat, garnish with fresh parsley, and pour over chicken,or vegetables.