How to Make It
Place the mushroom on a foil-lined baking sheet, spray twice with oil, and season with salt and pepper. Slide under a medium-hot broiler for 5-6 minutes until cooked and still juicy. Keep warm.
Meanwhile, in a small bowl, combine the ground beef with the steak rub, and gently form into a loose burger. Spray a small frying pan with 3 squirts of oil, and cook the burger over medium heat for 2–3 minutes each side for medium-rare, or until done to your liking.
Serve the burger on top of the mushroom, topped with the onion rings and arugula.
Smart Tip: Try not to compact the meat too much when making the burger—a loosely textured burger will look bigger and taste more tender and juicy.