Heat oil in a large cast-iron pan on medium-high heat. Add clusters of mushrooms and place a weight, such as a cast-iron press or another pan, on top. (If using large mushrooms, slice into 1/2-inch steaks or leave mushrooms whole.) Cook for 3 minutes, until golden brown. Flip mushrooms, replace weight, and cook for 2–3 minutes more. Remove the press from the mushrooms and season lightly with salt and pepper.

Serve with chimichurri drizzled on top.