In a large saute pan or cast iron skillet or two pans if you only have 10 inch saute pans, heat the oil until hot and shimmering.
Add the mushrooms and cook until caramelized on medium-high heat Season to taste with salt and pepper, then add the butter, garlic and parsley and cook for 1 minute more, stirring to distribute the seasonings.
Double check the seasoning for salt and adjust if needed, then remove the mushrooms from the pan with a slotted spoon to remove any excess fat, or allow them to dry on paper towels for a second, or just serve it on paper towels.
Cleaning different species of mushrooms will take the most time with this dish. We clean hundreds of pounds a year at the Salt Cellar by dipping them in water if needed to loosen grit, and trimming with a paring knife, or rubbing with a cloth depending on species. Afterwords we dry them overnight wrapped or covered in thick towels in the fridge.