Prepare the Station : Wipe mushrooms clean with a damp paper towel and cut them into thick, uniform slices, about ⅓-inch thick. Thinly slice the garlic cloves and roughly chop the parsley. Set everything aside within easy reach of the stovetop.
Heat the Pan : Place the cast-iron skillet over medium-high heat. Add approximately 3 tablespoons of the olive oil and heat until it shimmers, about 2 minutes. A hot pan is crucial for achieving a golden-brown sear.
Develop the Sear : Carefully arrange half of the sliced mushrooms in a single layer in the skillet. Let them cook undisturbed for 4–5 minutes, until the undersides are deeply browned. Stir and continue cooking for another 3–4 minutes until they are tender and golden all over. Transfer the first batch to a plate.
Bloom the Aromatics : Reduce the heat to medium. Add the remaining olive oil to the skillet along with the sliced garlic and the whole sprig of rosemary. Cook for just 45–60 seconds, stirring constantly, until the garlic is fragrant and turns a pale golden color.
Unify and Glaze : Return the seared mushrooms to the skillet with the garlic and rosemary. Season with the kosher salt, black pepper, and a pinch of red pepper flakes. Pour in the balsamic vinegar to deglaze the pan, scraping up any browned bits from the bottom.
Final Infusion : Toss everything together and cook for another 2 minutes to allow the flavors to meld completely. The mushrooms should be glossy and well-coated. Remove the pan from the heat and discard the rosemary sprig.
Garnish and Serve : Finish the dish with a squeeze of fresh lemon juice and a final drizzle of any remaining olive oil. Sprinkle with the fresh, chopped parsley and serve immediately.
