In a large skillet, melt the butter over medium heat. Once melted and bubbling, add the finely chopped onion. Sauté for approximately 4-5 minutes, or until the onion becomes translucent and soft.
Incorporate the minced garlic into the skillet, cooking for an additional minute or until the garlic becomes fragrant, ensuring not to let it brown.
Introduce the sliced mushrooms to the skillet, stirring well to combine. Sauté the mushrooms for about 7-8 minutes, or until they lose moisture and develop a golden-brown color.
Carefully pour in the vegetable broth and raise the heat slightly to bring the mixture to a gentle simmer. Let it simmer for about 5 minutes, allowing the liquid to reduce and concentrate in flavor.
Once reduced, lower the heat to medium-low and stir in the heavy cream and Dijon mustard, blending everything together until smooth and even.
Sprinkle in the dried thyme and smoked paprika, then season generously with salt and freshly ground black pepper to taste. Let the sauce simmer for an additional 5-7 minutes, stirring occasionally, until it thickens to your desired consistency.
After thickening, remove the skillet from the heat. Garnish the sauce generously with freshly chopped parsley just before serving.
Notes
Serve over grilled chicken, sautéed vegetables, or roasted potatoes. Pair with spaetzle or mashed potatoes.