For the Stuffed Flank Steak:
2 Using a sharp knife, butterfly the flank steak lengthwise, parallel with the fibers of the meat, making sure to stop just before slicing through the opposite side. Open the steak up like a book and season the sliced side with salt, pepper, smoked paprika and rosemary. Cover the steak with plastic wrap or butcher paper and flatten it with a meat mallet or rolling pin until it’s around 1/4-inch thick all over. Transfer the steak to the fridge, covered, and let marinate 1 to 4 hours.
3 Preheat a large, oven-safe skillet over medium heat. Add the olive oil and heat until shimmering. Add the mushrooms and cook, undisturbed, until the bottom sides are golden brown, around 3 minutes. Add the shallots, garlic and chiles and lightly season with salt and pepper to taste. Cook, stirring, until fragrant, around 1 minute. Add the spinach and cook, stirring, until the leaves have significantly wilted and there is very little moisture remaining in the pan, around 3 minutes. Taste the mixture for seasoning and adjust with salt and pepper as desired. Transfer to a bowl and let cool completely.
4 Remove the steak from the fridge and preheat your Traeger grill or oven to 400F. If using a conventional grill, setup a two-zone heat for indirect cooking.
5 Spread the vegetable mixture evenly over the flank steak. Position the Beemster gouda sticks along the narrower end of the steak and tightly roll the steak with the grain starting at the cheese side. Using butcher’s twine, tie the steak every 2 inches and once along the length to hold everything in place.
6 Wipe the same oven-safe skillet clean and place over medium-high heat. Add the avocado oil and heat until shimmering. Carefully add the steak to the pan and brown on all sides, around 12 minutes total. Transfer the pan to the preheated grill or oven and cook until the thickest part of the steak registers 135 F for medium-rare and the cheese has melted in the center (you can check for resistance with a wooden skewer or meat probe). Transfer the steak to a board and cover with foil to keep warm while you prepare the Mushroom Cream Sauce.
For the Mushroom Cream Sauce:
9 Place the same pan used to cook the steak over a medium heat. Add the butter and heat until melted. Add the mushrooms and cook, undisturbed, until golden brown, around 3 minutes. Add the shallots and garlic and season with a pinch of salt and pepper. Cook, stirring, until fragrant, 1 minute. Add the beef stock and mustard, stir until combined and cook until the liquid has reduced by ¼ in volume. Add the cream and stir to combine. Taste for seasoning and adjust with salt and pepper as desired.
10 Slice the flank steak in between each piece of twine. Transfer to a serving platter and spoon the Mushroom Cream Sauce overtop. Serve with side dish of choice, such as mashed potatoes, sautéed vegetables or grilled vegetables.
