Crisp the Potatoes: Heat large skillet on medium high heat and then add oil. Place smashed potatoes in single layer on the pan and cook until both sides are crispy.
Make the Gravy: In saucepan, melt 2 Tablespoons of butter over medium-high heat. Add mushrooms and cook until golden and excess mushroom liquid evaporates, about 10 minutes.
Add garlic and remaining butter (4 tablespoons). Cook for about 1 minute.
When gravy starts to boil, reduce heat to low, stirring often, simmer until thickened, about 10-20 minutes depending on your preferred thickness. Check for seasoning and salt again if needed.