Prepare the Mashed Potatoes:
1. Place the peeled and quartered potatoes in a large pot and cover with water. Add salt and bring to a boil.
2. Reduce heat and simmer until potatoes are tender, about 20 minutes.
3. Drain the potatoes and return them to the pot. Add the milk and butter, and mash until smooth and creamy. Season with salt and pepper to taste. Cover and keep warm.

Bake the Chicken Tenderloin:
1. Preheat the oven to 400 F (200 C).
2. Season the chicken tenderloins with salt and pepper. In a large ovenproof skillet, heat olive oil over medium-high heat.
3. Sear the tenderloins on both sides until browned, about 3 minutes per side. Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until cooked through.

Prepare the Mushroom Gravy:
1. In a separate skillet, saut the sliced mushrooms, diced onion, and minced garlic over medium heat until the mushrooms are browned, about 5 minutes.
2. Sprinkle the flour over the mushroom mixture and cook, stirring, for 2 minutes to eliminate the raw flour taste.
3. Gradually stir in the chicken broth and Worcestershire sauce, scraping up any browned bits from the bottom of the skillet. Bring to a simmer, and let the gravy thicken, about 5 minutes. Stir in parsley if using.
4. Adjust seasoning with salt and pepper to taste.

Serve:
1. Spoon the creamy mashed potatoes onto plates or bowls.
2. Place a baked chicken tenderloin over the mashed potatoes.
3. Ladle the mushroom gravy over the chicken and potatoes.
4. Serve hot, garnished with additional parsley if desired.