Pre-heat oven to 400°F.
In a pan, over medium high, heat olive oil and sauté mushrooms for 3 to 4 minutes until they start to release their juices.
Add red onion, red chili flakes and salt; cook for another 3 to 4 minutes, until most of the juices evaporate.
While vegetables are cooking, add pre-made flatbread/pizza to the oven and cook for 4 to 6 minutes; until crust is slightly toasted.
Cook the streak, to your desired doneness, on a grill or in a cast iron skillet on thestovetop; cooking times will vary depending on cut and thickness.
Remove to a plate and tent with aluminum foil for 5 to 10 minutes before slicing; temperature will continue to rise as it rests.
Assemble the flatbread with sautéed mushrooms and onions, followed by the steak.
Add cherry tomato halves and top with arugula and gorgonzola; serve immediately.
Drizzle top with balsamic vinegar, if desired.