In a medium skillet, melt butter over medium heat. Add the diced shallot and cook for 2–3 minutes to soften. Add the garlic and sauté for 30 seconds until fragrant.
Pour in the cream sherry and stir. Scrape up any bits from the bottom. Simmer for 3–5 minutes to reduce slightly and cook off the alcohol.
Lower the heat and stir in the heavy cream and broth. Simmer gently for 5–7 minutes. Stir occasionally, until the sauce thickens and coats the back of a spoon.
Season with salt and freshly ground pepper to taste. Add Dijon mustard and grated Parmesan if using. Stir until smooth.
Remove from heat and garnish with fresh herbs. Serve warm over your favorite proteins, pasta, or vegetables.
