Place a medium to large size ovenproof skillet over medium heat. Pan size depends on how deep you’d like your frittata. Add a drizzle of EVOO one time around the pan. Add chopped onion and Swiss chard stems and toss to coat in oil. Cook for about a minute. Add the roughly chopped mushrooms and toss. Cook until the veggies are tender, about 3-4 minutes. If they are browning, turn down the flame.