On the morning of the day you’d like to cook this steak, remove it from the plastic and defrost it on a plate in the fridge. Remove the steak from the fridge at least half an hour before cooking.
Add parsley, garlic, red wine vinegar, 1 tablespoon lemon juice, shallot and ¾ cup olive oil to a food processor or blender and pulse until finely chopped, stopping to scrape down the sides as necessary. Season with salt and pepper, to taste and set aside.
Five minutes before you cook the steak, season generously with kosher salt and freshly ground black pepper. Heat a cast iron or other heavy bottomed pan over medium high heat. Sear the steak for 3-4 minutes per side until the steak is medium rare. Let the steak rest for 10 minutes before slicing.
While the steak is resting you can cook the mushrooms. Wipe away any dirt with a dry paper towel. You don’t need to wash them. Cut the mushrooms in half from top to bottom. Heat the same pan you used for the steak over medium high heat and add two or three tablespoons of olive oil. Add the mushrooms to the pan, cut side down. Season with salt and pepper and sauté for three to four minutes on that side until lightly caramelized. Flip the mushrooms over and cook for another minute or two until the mushrooms are tender.
Slice the steak crosswise, against the grain of the meat. Arrange the slices of steak on a platter with the mushrooms. Scatter the sunflower shoots over them. Season with a drizzle of olive oil, a squeeze of lemon juice and some flaky sea salt. Serve with Chimichurri sauce.