Trim away any dirty or woody base, then cut the lion’s mane top to bottom into 3–3.5 cm thick slabs.
Place the steaks in a dry, cold pan over medium heat. Press with a second pan and cook 3–4 minutes per side until steam subsides and the surface looks dry and matte. Drain excess water if they are especially waterlogged.
Remove the mushrooms from the pan. Increase heat to high, add the oil, and heat until shimmering.
Return the steaks to the pan and sear 4–5 minutes on the first side until golden. Flip and sear 4–5 minutes on the second side.
Reduce the heat slightly. Sprinkle a light, even layer of the blackening rub over the top. Flip and sear 1 minute, then season the second side, flip again, and sear for a further 1 minute until the spices are toasted and aromatic.
Lower the heat to medium. Add the butter, garlic, and thyme (if using). Baste the steaks with the melted butter for 30–60 seconds.
Rest for 1 minute before serving.