Prepare pork tenderloin by slicing into ½ inch medallions. Season the flour with 1 teaspoon salt, pepper, garlic powder, and nutmeg. Then dredge the medallions through the flour.
In a large saucepan over medium-high heat, add the olive oil. Once shimmering, add the pork medallions in one layer and sear on both sides for 1-2 minutes until golden brown. Remove and set the medallions aside.
Lower the heat to medium-low. Add shallots to the pan and saute for 3 minutes until the shallots begin to soften. Stir in the mushrooms and saute for 6-8 minutes until mushrooms begin to brown. Season with ½ teaspoon of salt.
Pour in the beef stock and marsala wine. Use a wooden spoon to deglaze the bottom of the pan. Bring to a simmer and simmer for 3-5 minutes until the sauce is reduced by half or the fragrance of alcohol dissipates.
Return the pork tenderloin medallions to the pan and simmer in the sauce for 5 minutes or until the pork reaches 145℉.
Remove the pork from the pan and pour the sauce over the pork tenderloin medallions to serve.
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Notes
Always dredge tenderloin medallions in flour immediately before searing. Flour gets absorbed into meat if it sits for too long.
Before searing, wait till the pan gets hot enough that you hear a sizzle when you place the pork in. You want to sear quickly so that you don't cook the pork all the way through, but also get a crust on the pork.
Don't season the mushrooms with salt until they have begun to brown. If you season mushrooms with salt too soon, they will hold onto more liquid.
After you remove the pork, simmer the sauce until thick enough that when you run a spoon through it, it leaves a trail.