How to Make It
In a 12-inch ovenproof skillet, heat the grapeseed oil until shimmering. Season the pork with salt and pepper and cook over moderately high heat until browned all over, about 5 minutes. Transfer to the oven and roast the pork until an instant-read thermometer inserted in the thickest part registers 135°, 18 to 20 minutes.
Meanwhile, in a very large skillet, melt 1 tablespoon of the butter in the olive oil. Add the mushrooms, season with salt and pepper and cook over moderately high heat, turning occasionally, until golden, about 15 minutes. Add the minced garlic and the parsley and cook, stirring, until the garlic is fragrant, about 30 seconds. Remove from the heat.
Set the pork over moderate heat. Add the remaining 3 tablespoons of butter, the crushed garlic and the thyme to the skillet and cook until the butter is foamy. Baste the meat with the butter just until the butter browns, 2 to 3 minutes.
Transfer the pork to a cutting board and cut into thick slices. Spoon the mushrooms onto a platter, top with the pork and drizzle with the brown butter. Garnish with walnuts, shallot, chives and flaky salt and serve.