In a medium skillet, heat 2 tablespoons of olive oil and 1 tablespoon of unsalted butter over medium-high heat until the butter is melted and shimmering. Add 8 ounces of sliced cremini mushrooms and ½ medium chopped onion. Sauté for 10-12 minutes, stirring occasionally, until the mushrooms release their moisture and the vegetables are nicely browned.

Reduce the heat to medium and stir in 1 minced garlic clove and ½ teaspoon of Dijon mustard. Cook for about 30 seconds, stirring constantly, until the garlic becomes fragrant.

Add ⅓ cup of marsala wine to the skillet, scraping the bottom to deglaze and incorporate all the flavorful bits. Allow the mixture to bubble for 30 seconds to 1 minute.

Pour in 1 cup of heavy cream, stirring well to combine it with the marsala wine mixture. Increase the heat slightly and let the sauce simmer gently for about 2-3 minutes.

Create a cornstarch slurry by mixing ¼ teaspoon of cornstarch with a little cold water until smooth. Gradually stir this into the simmering sauce, and cook for another 5 minutes over medium heat, stirring often.

Taste your creamy Easy Marsala Sauce and season with salt and pepper to your liking. Stir in some chopped fresh parsley for a burst of color and freshness. Serve the sauce immediately over grilled chicken, steak, or roasted vegetables.