In the same pan, add another Tbs. of EVOO, add diced onion over medium low heat, add salt and pepper. Using a wooden spoon, scrape up the browned bits on the bottom of the pan and cook onions about 5 minutes until translucent. Add bell pepper and continue to saute over medium heat for 3 more minutes. Finally, add the sliced mushrooms, stir to combine and cook another 3 minutes.