• Preheat the oven to Hi-Broil and grease a broiling pan with baking spray.
• Place the steak in shallow bowl and season generously with salt & pepper. Add the balsamic vinegar, brown sugar and minced garlic and turn the steaks over a few times to coat. (The steaks can marinate like this for 1 hour up to overnight if you have the extra time. Otherwise, you can cook them immediately too).
• Broil the steak for 10-12 minutes until medium-rare. Remove from the oven and let sit.
• While the steak rests, turn the oven to 400 degrees, grease a baking sheet and place the portobello caps on the sheet. Drizzle with the olive oil and season with salt & pepper.
• Bake the portobellos for 10 minutes until they start to shrink and expel their water.
• While the mushrooms cook, slice the steak into thin strips.
• Remove the mushrooms from the oven, drain the water from the caps and set aside.

For the Pistachio Pesto

• Combine the basil, pistachios, parmesan, garlic, lemon juice and salt & pepper in a food processor.
• Process and drizzle the olive oil in with the processor running. Scrape down the sides a few times until creamy.

To Assemble

• Place a handful of arugula on top of each mushroom cap.
• Top with sliced steak and drizzle the pesto over the top.
• Garnish with chopped pistachios.