Instructions:
- Gently wipe the mushroom caps clean. Snap off the stems and use a small spoon to gently scrape out the dark gills to prevent excess moisture during cooking.
- In a bowl, thoroughly whisk together the balsamic vinegar, olive oil, soy/tamari, minced garlic, thyme, smoked paprika, and black pepper to create the marinade.
- Place the cleaned mushroom caps in a shallow dish or zip-top bag. Pour the marinade over them, ensuring both sides are coated. Marinate at room temperature for at least 30 minutes, or refrigerate for up to 4 hours.
- Remove mushrooms from the marinade, allowing excess liquid to drip off. Crucially, pat the caps gently with kitchen paper until they feel relatively dry.
- Preheat a large skillet or cast-iron pan over medium-high heat. Add a small drizzle of oil. Place the mushrooms gill-side down first and cook undisturbed for 4–5 minutes until a deep, rich brown crust forms.
- Flip the mushrooms. Add the butter and the rosemary sprig to the pan. Once the butter melts and foams, tilt the pan slightly and use a spoon to continuously baste the foaming butter over the top of the mushrooms for another 3–4 minutes until tender.
- Remove the Portobello steaks from the pan immediately. Let them rest on a cutting board for 3 minutes, then serve whole or sliced, finished with a sprinkle of flaky sea salt.