If your 1.5-pound flank steak is not already butterflied, butterfly it by slicing horizontally almost all the way through so it opens like a book; then place it between plastic and gently pound with a meat mallet to an even thickness. Pat dry and season both sides generously with salt and freshly ground pepper. Set aside.

Preheat the oven to 350°F (175°C).

Heat 1 tablespoon olive oil in a sauté pan over medium heat. Add 4 ounces chopped mushrooms, 1 small minced shallot, and ¼ teaspoon fresh thyme. Sprinkle with salt and cook, stirring occasionally, until the mushrooms and shallot are softened and most liquid has evaporated.

Add 2 cups packed baby spinach to the pan and cook just until wilted. Transfer the mixture to a bowl and let it cool enough so it will not melt the cheese when assembled.

Lay the butterflied flank steak flat on a work surface with the grain running left to right. Arrange the 8 ounces sliced Provolone cheese over the beef, leaving a 1-inch border all the way around. Spread the cooled spinach–mushroom mixture evenly over the Provolone.

Starting at the edge closest to you, roll the steak away from you into a tight log. Tie the roulade with butcher’s twine at 2-inch intervals to hold its shape.

Place the tied roulade seam-side down in a greased baking dish or on a rack in a roasting pan. Roast in the preheated oven for 45 minutes to 1 hour, or until an instant-read thermometer inserted into the center registers about 130°F for medium-rare (cook to your desired doneness).

Remove the roulade from the oven, tent loosely with foil, and let it rest for 10 minutes. Cut and remove the twine, then slice into rounds to serve.