In a large skillet over medium heat, melt the butter and add the thinly sliced onion. Cook for about 1½ minutes, just until the onion softens and becomes translucent.

Add the sliced mushrooms to the pan and stir to combine. Increase the heat to medium-high. Cook, stirring every minute or so, until the mushrooms are golden and the onions are nicely caramelized. This should take about 8 minutes. Halfway through, season the mix with a bit of salt and pepper to draw out moisture and deepen the flavor.

Reduce the heat to medium-low and stir in the minced garlic. Cook for about 1 minute, just until the garlic becomes fragrant—be careful not to let it burn.

Pour in the stock to deglaze the pan, scraping up all the flavorful brown bits from the bottom. Let the liquid reduce by about half—this should take approximately 1½ minutes.

Lower the heat to low and stir in the heavy cream and Dijon mustard. Mix until fully combined and creamy. Taste and adjust seasoning with more salt and pepper if needed.

In a small bowl, mix the cornstarch with 2 tablespoons of cold water. Stir well to form a smooth slurry, then pour it into the sauce. Allow the mixture to simmer gently for about 3 minutes, stirring regularly, until it thickens to your desired consistency.

Remove from heat and finish with a sprinkle of finely chopped parsley or thyme leaves. Serve hot.