Heat 2 tbsp olive oil in a large skillet over medium-high. Add cauliflower, season with salt and pepper, and cook undisturbed for 4 to 5 minutes until browned on the bottom. Stir and cook another 3 to 4 minutes until tender and charred in spots. Transfer to a plate.

In the same skillet, add the remaining 1 tbsp oil and the mushrooms. Spread in a single layer, cook 3 to 4 minutes without stirring, then stir and cook until golden and the liquid evaporates, about 2 to 3 minutes more.

Return cauliflower to the pan, add garlic and butter. Cook 1 to 2 minutes, stirring, until the garlic is fragrant and butter melts.

Remove from heat, stir in parsley and lemon juice. Taste, adjust salt and pepper, and serve immediately.